feat(neptune): add engine version 1.1.0.0 and instance types t4g, r6g (#17669)

Add new instance types general-purpose T4g and memory-optimized R6g (see [announcement](https://aws.amazon.com/about-aws/whats-new/2021/11/aws-graviton2-based-instances-amazon-neptune/)). The specific instance types were copied from the [pricing page](https://aws.amazon.com/de/neptune/pricing/).

Add new Neptune engine version 1.1.0.0 which was part of the announcement, too.

Deployment tested successfully in region us-east-1:

```ts
new neptune.DatabaseCluster(this, 'Database', {
  vpc,
  instanceType: neptune.InstanceType.T4G_MEDIUM,
  engineVersion: neptune.EngineVersion.V1_1_0_0,
});

new neptune.DatabaseCluster(this, 'Database2', {
  vpc,
  instanceType: neptune.InstanceType.R6G_LARGE,
  engineVersion: neptune.EngineVersion.V1_1_0_0,
});
```


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: jumic

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

@@ -50,6 +50,10 @@ export class EngineVersion {
    * Neptune engine version 1.0.5.0
    */
   public static readonly V1_0_5_0 = new EngineVersion('1.0.5.0');
+  /**
+   * Neptune engine version 1.1.0.0
+   */
+  public static readonly V1_1_0_0 = new EngineVersion('1.1.0.0');
 
   /**
    * Constructor for specifying a custom engine version

packages/@aws-cdk/aws-neptune/lib/instance.ts

@@ -12,6 +12,46 @@ import { IParameterGroup } from './parameter-group';
  */
 export class InstanceType {
 
+  /**
+   * db.r6g.large
+   */
+  public static readonly R6G_LARGE = InstanceType.of('db.r6g.large');
+
+  /**
+   * db.r6g.xlarge
+   */
+  public static readonly R6G_XLARGE = InstanceType.of('db.r6g.xlarge');
+
+  /**
+   * db.r6g.2xlarge
+   */
+  public static readonly R6G_2XLARGE = InstanceType.of('db.r6g.2xlarge');
+
+  /**
+   * db.r6g.4xlarge
+   */
+  public static readonly R6G_4XLARGE = InstanceType.of('db.r6g.4xlarge');
+
+  /**
+   * db.r6g.8xlarge
+   */
+  public static readonly R6G_8XLARGE = InstanceType.of('db.r6g.8xlarge');
+
+  /**
+  * db.r6g.12xlarge
+  */
+  public static readonly R6G_12XLARGE = InstanceType.of('db.r6g.12xlarge');
+
+  /**
+   * db.r6g.16xlarge
+   */
+  public static readonly R6G_16XLARGE = InstanceType.of('db.r6g.16xlarge');
+
+  /**
+   * db.t4g.medium
+   */
+  public static readonly T4G_MEDIUM = InstanceType.of('db.t4g.medium');
+
   /**
    * db.r5.large
    */