fix(rds): secret for ServerlessCluster is not accessible programmatically (aws#10657)

In a last-minute change, the secret was changed from public (matching all of the
other RDS constructs) to private. This means users who don't specify a secret,
and rely on the auto-generated one, have no programmatic means of accessing the
secret.

Brought up by a user on the CDK.dev Slack channel.


----

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

Author: njlynch

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

@@ -319,15 +319,16 @@ export class ServerlessCluster extends ServerlessClusterBase {
   /**
    * The secret attached to this cluster
    */
-  private readonly secret?: secretsmanager.ISecret;
+  public readonly secret?: secretsmanager.ISecret;
+
   private readonly subnetGroup: ISubnetGroup;
   private readonly vpc: ec2.IVpc;
   private readonly vpcSubnets?: ec2.SubnetSelection;
 
   private readonly singleUserRotationApplication: secretsmanager.SecretRotationApplication;
   private readonly multiUserRotationApplication: secretsmanager.SecretRotationApplication;
 
-  constructor(scope:Construct, id: string, props: ServerlessClusterProps) {
+  constructor(scope: Construct, id: string, props: ServerlessClusterProps) {
     super(scope, id);
 
     this.vpc = props.vpc;