feat(neptune-alpha): specify port for the cluster (aws#31137)

### Issue # (if applicable)

Closes aws#31074.

### Reason for this change

Cloudformation supports for setting port number for the Neptune cluster but AWS CDK cannot do this.

### Description of changes

Add `port` prop to `DatabaseClusterProps`.

### Description of how you validated changes

Added both unit and integ tests.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

packages/@aws-cdk/aws-neptune-alpha/README.md

@@ -138,6 +138,18 @@ new neptune.DatabaseCluster(this, 'Cluster', {
 });
 ```
 
+## Port
+
+By default, Neptune uses port `8182`. You can override the default port by specifying the `port` property:
+
+```ts
+const cluster = new neptune.DatabaseCluster(this, 'Database', {
+  vpc,
+  instanceType: neptune.InstanceType.R5_LARGE,
+  port: 12345,
+});
+```
+
 ## Logging
 
 Neptune supports various methods for monitoring performance and usage. One of those methods is logging

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

@@ -332,6 +332,13 @@ export interface DatabaseClusterProps {
    * @default - false
    */
   readonly copyTagsToSnapshot?: boolean;
+
+  /**
+   * The port number on which the DB instances in the DB cluster accept connections.
+   *
+   * @default 8182
+   */
+  readonly port?: number;
 }
 
 /**
@@ -617,6 +624,7 @@ export class DatabaseCluster extends DatabaseClusterBase implements IDatabaseClu
       deletionProtection: deletionProtection,
       associatedRoles: props.associatedRoles ? props.associatedRoles.map(role => ({ roleArn: role.roleArn })) : undefined,
       iamAuthEnabled: Lazy.any({ produce: () => this.enableIamAuthentication }),
+      dbPort: props.port,
       // Backup
       backupRetentionPeriod: props.backupRetention?.toDays(),
       preferredBackupWindow: props.preferredBackupWindow,

packages/@aws-cdk/aws-neptune-alpha/test/cluster.test.ts

@@ -221,6 +221,24 @@ describe('DatabaseCluster', () => {
     });
   });
 
+  test('cluster with port', () => {
+    // GIVEN
+    const stack = testStack();
+    const vpc = new ec2.Vpc(stack, 'VPC');
+
+    // WHEN
+    new DatabaseCluster(stack, 'Database', {
+      vpc,
+      instanceType: InstanceType.R5_LARGE,
+      port: 1234,
+    });
+
+    // THEN
+    Template.fromStack(stack).hasResourceProperties('AWS::Neptune::DBCluster', {
+      DBPort: 1234,
+    });
+  });
+
   test('cluster with imported parameter group', () => {
     // GIVEN
     const stack = testStack();