[core-amqp][event-hubs] support specifying a custom endpoint (#12909)

Fixes #12901 

### Purpose of change

Some users run their applications in a private network that lacks access to public endpoints, such as <namespace>.servicebus.windows.net. Due to these restrictions, these applications are unable to create direct connections to the Azure Event Hubs service using the standard endpoint address and require the ability to specify a custom host name to ensure they route through the proper intermediary for the connection to be made.

### Testing

I've added tests that verify the connection has been configured with the custom endpoint correctly.

I also manually ran these changes by setting up an Application Gateway in Azure with:
- backend pool with a backend target that points to the FQDN of my Event Hubs Namespace (`<namespace>.servicebus.windows.net`)
- A front-end listener using a HTTPS and a nonstandard port (200).
- HTTP settings with the following:
   - backend port: 443
   - Override with new host name: set to 'Yes'
- A rule that maps the front-end listener to the back-end target and uses the HTTP settings above.

I then had a test script that sent events, received events, and called operations on the $management link setting the `customEndpointAddress` to the public IP address from my application gateway.

Here's a simplified version of that script:
```js
const { EventHubProducerClient } = require('@azure/event-hubs');
const ws = require('ws');
const connectionString = `Endpoint=sb://<namespace>.servicebus.windows.net/;SharedAccessKeyName=sakn;SharedAccessKey=key`;
const eventHubName = `my-hub`;

async function run() {
  const client = new EventHubProducerClient(connectionString, eventHubName, {
    customEndpointAddress: `https://my.ip.address:200`,
    webSocketOptions: {
      webSocket: ws
    }
  });

  const props = await client.getEventHubProperties();
  console.log(props);
  return client.close();
}
run().catch(console.error);
```

Note that in my application gateway I also used self-signed certs so needed to do some additional work so node.js would recognize my certificate, but that's separate from the customEndpointAddress work and doesn't require changes from our SDK.

sdk/core/core-amqp/CHANGELOG.md

@@ -1,8 +1,15 @@
 # Release History
 
-## 2.0.1 (Unreleased)
+## 2.1.0 (Unreleased)
 
-- Fixes the bug reported in issue [12610](https://github.com/Azure/azure-sdk-for-js/issues/12610).
+- Adds the ability to configure the `amqpHostname` and `port` that a `ConnectionContextBase` will use when connecting to a service.
+  The `host` field refers to the DNS host or IP address of the service, whereas the `amqpHostname`
+  is the fully qualified host name of the service. Normally `host` and `amqpHostname` will be the same.
+  However if your network does not allow connecting to the service via the public host,
+  you can specify a custom host (e.g. an application gateway) via the `host` field and continue
+  using the public host as the `amqpHostname`.
+
+* Fixes the bug reported in issue [12610](https://github.com/Azure/azure-sdk-for-js/issues/12610).
   Previously, `retry` would still sleep one more time after all retry attempts were exhausted before returning.
   Now, `retry` will return immediately after all retry attempts are completed as necessary.
 

sdk/core/core-amqp/package.json

@@ -1,7 +1,7 @@
 {
   "name": "@azure/core-amqp",
   "sdk-type": "client",
-  "version": "2.0.1",
+  "version": "2.1.0",
   "description": "Common library for amqp based azure sdks like @azure/event-hubs.",
   "author": "Microsoft Corporation",
   "license": "MIT",

sdk/core/core-amqp/review/core-amqp.api.md

@@ -159,10 +159,12 @@ export enum ConditionErrorNameMapper {
 
 // @public
 export interface ConnectionConfig {
+    amqpHostname?: string;
     connectionString: string;
     endpoint: string;
     entityPath?: string;
     host: string;
+    port?: number;
     sharedAccessKey: string;
     sharedAccessKeyName: string;
     webSocket?: WebSocketImpl;

sdk/core/core-amqp/src/ConnectionContextBase.ts

@@ -122,9 +122,9 @@ export const ConnectionContextBase = {
     const connectionOptions: ConnectionOptions = {
       transport: Constants.TLS,
       host: parameters.config.host,
-      hostname: parameters.config.host,
+      hostname: parameters.config.amqpHostname ?? parameters.config.host,
       username: parameters.config.sharedAccessKeyName,
-      port: 5671,
+      port: parameters.config.port ?? 5671,
       reconnect: false,
       properties: {
         product: parameters.connectionProperties.product,
@@ -147,10 +147,11 @@ export const ConnectionContextBase = {
       const host = parameters.config.host;
       const endpoint = parameters.config.webSocketEndpointPath || "";
       const socketOptions = parameters.config.webSocketConstructorOptions || {};
+      const port = parameters.config.port ?? 443;
 
       connectionOptions.webSocketOptions = {
         webSocket: socket,
-        url: `wss://${host}:443/${endpoint}`,
+        url: `wss://${host}:${port}/${endpoint}`,
         protocol: ["AMQPWSB10"],
         options: socketOptions
       };

sdk/core/core-amqp/src/connectionConfig/connectionConfig.ts

@@ -26,9 +26,22 @@ export interface ConnectionConfig {
    */
   endpoint: string;
   /**
-   * @property {string} host - The host "<yournamespace>.servicebus.windows.net".
+   * The DNS hostname or IP address of the service.
+   * Typically of the form "<yournamespace>.servicebus.windows.net" unless connecting
+   * to the service through an intermediary.
    */
   host: string;
+  /**
+   * The fully qualified name of the host to connect to.
+   * This field can be used by AMQP proxies to determine the correct back-end service to
+   * connect the client to.
+   * Typically of the form "<yournamespace>.servicebus.windows.net".
+   */
+  amqpHostname?: string;
+  /**
+   * The port number.
+   */
+  port?: number;
   /**
    * @property {string} connectionString - The connection string.
    */
@@ -135,7 +148,7 @@ export const ConnectionConfig = {
       throw new TypeError("Missing 'entityPath' in configuration");
     }
     if (config.entityPath != undefined) {
-       config.entityPath = String(config.entityPath);
+      config.entityPath = String(config.entityPath);
     }
 
     if (!isSharedAccessSignature(config.connectionString)) {