Provide config service methods to java clients (#3532)

Refactors existing java-client gRPC response observers into one shared
implementation, so that all can be canceled.

Fixes #3528

java-client/session/src/main/java/io/deephaven/client/impl/ConfigService.java

@@ -0,0 +1,24 @@
+package io.deephaven.client.impl;
+
+import io.deephaven.proto.backplane.grpc.ConfigValue;
+
+import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+
+/**
+ * Provides server-specified configuration values to gRPC clients.
+ */
+public interface ConfigService {
+    /**
+     * Returns constants from the server that may be helpful to correctly authenticate with the server. As such,
+     * authentication is not required to obtain these values.
+     */
+    CompletableFuture<Map<String, ConfigValue>> getAuthenticationConstants();
+
+    /**
+     * Returns constants from the server that are specified as being appropriate for clients to read. By default these
+     * include values like the suggested authentication token refresh interval, and the server-side version of
+     * deephaven, barrage, and java.
+     */
+    CompletableFuture<Map<String, ConfigValue>> getConfigurationConstants();
+}

java-client/session/src/main/java/io/deephaven/client/impl/Session.java

@@ -11,7 +11,8 @@
  * A session represents a client-side connection to a Deephaven server.
  */
 public interface Session
-        extends AutoCloseable, ApplicationService, ConsoleService, InputTableService, ObjectService, TableService {
+        extends AutoCloseable, ApplicationService, ConsoleService, InputTableService, ObjectService, TableService,
+        ConfigService {
 
     // ----------------------------------------------------------