feat(rds): introduce type-safe engine versions

Change engine versions, for boh cluster and instance engines,
from strings to strongly-typed classes with static constants representing common versions.

Fixes aws#6532

BREAKING CHANGE: the property 'version' has been changed from string to an engine-specific
  version class; use VersionClass.of() if you need to create a specific version of an engine from a string
* **rds**: the property engineVersion in IClusterEngine changed from an optional string to a required EngineVersion
* **rds**: the property engineVersion in IInstanceEngine changed from an optional string to a required EngineVersion

packages/@aws-cdk/aws-rds/README.md

@@ -48,14 +48,14 @@ use the static factory methods on `DatabaseClusterEngine`:
 ```typescript
 new rds.DatabaseCluster(this, 'Database', {
   engine: rds.DatabaseClusterEngine.aurora({
-    version: '5.6.mysql_aurora.1.17.9',
+    version: rds.AuroraEngineVersion.VER_1_17_9, // different version class for each engine type
   },
   ...
 })
 ```
 
-See [the AWS documentation](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-rds-database-instance.html#cfn-rds-dbinstance-engineversion)
-for a list of the supported versions for each engine type.
+If there isn't a constant for the exact version you want to use,
+all of the `Version` classes have a static `of` method that can be used to create an arbitrary version.
 
 By default, the master password will be generated and stored in AWS Secrets Manager with auto-generated description.
 
@@ -87,12 +87,15 @@ use the static factory methods on `DatabaseInstanceEngine`:
 ```typescript
 const instance = new rds.DatabaseInstance(this, 'Instance', {
   engine: rds.DatabaseInstanceEngine.oracleSe1({
-    version: '19.0.0.0',
+    version: rds.OracleEngineVersion.VER_19, // different version class for each engine type
   }),
   ...
 });
 ```
 
+If there isn't a constant for the exact version you want to use,
+all of the `Version` classes have a static `of` method that can be used to create an arbitrary version.
+
 To use the storage auto scaling option of RDS you can specify the maximum allocated storage.
 This is the upper limit to which RDS can automatically scale the storage. More info can be found
 [here](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PIOPS.StorageTypes.html#USER_PIOPS.Autoscaling)

packages/@aws-cdk/aws-rds/lib/cluster.ts

@@ -419,7 +419,7 @@ export class DatabaseCluster extends DatabaseClusterBase {
     const cluster = new CfnDBCluster(this, 'Resource', {
       // Basic
       engine: props.engine.engineType,
-      engineVersion: props.engine.engineVersion,
+      engineVersion: props.engine.engineVersion.fullVersion,
       dbClusterIdentifier: props.clusterIdentifier,
       dbSubnetGroupName: subnetGroup.ref,
       vpcSecurityGroupIds: securityGroups.map(sg => sg.securityGroupId),
@@ -495,7 +495,7 @@ export class DatabaseCluster extends DatabaseClusterBase {
       const instance = new CfnDBInstance(this, `Instance${instanceIndex}`, {
         // Link to cluster
         engine: props.engine.engineType,
-        engineVersion: props.engine.engineVersion,
+        engineVersion: props.engine.engineVersion.fullVersion,
         dbClusterIdentifier: cluster.ref,
         dbInstanceIdentifier: instanceIdentifier,
         // Instance properties

packages/@aws-cdk/aws-rds/lib/engine-version.ts

@@ -0,0 +1,23 @@
+/**
+ * A version of an engine -
+ * for either a cluster, or instance.
+ */
+export interface EngineVersion {
+  /**
+   * The full version string of the engine,
+   * for example, "5.6.mysql_aurora.1.22.1".
+   * It can be undefined,
+   * which means RDS should use whatever version it deems appropriate for the given engine type.
+   *
+   * @default - no version specified
+   */
+  readonly fullVersion?: string;
+
+  /**
+   * The major version of the engine,
+   * for example, "5.6".
+   * Used in specifying the ParameterGroup family
+   * and OptionGroup version for this engine.
+   */
+  readonly majorVersion: string;
+}

packages/@aws-cdk/aws-rds/lib/index.ts

@@ -1,3 +1,4 @@
+export * from './engine-version';
 export * from './cluster';
 export * from './cluster-ref';
 export * from './cluster-engine';