feat(sqs): add DLQ readonly property to Queue (aws#18232)

feat(@aws-cdk/aws-sqs): add DLQ readonly property

motivation is to be able to read the DLQ properties after the queue as been created. Such as referencing it in a different stack.

aws#18083  
----

*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-sqs/lib/queue.ts

@@ -320,6 +320,11 @@ export class Queue extends QueueBase {
    */
   public readonly fifo: boolean;
 
+  /**
+   * If this queue is configured with a dead-letter queue, this is the dead-letter queue settings.
+   */
+  public readonly deadLetterQueue?: DeadLetterQueue;
+
   protected readonly autoCreatePolicy = true;
 
   constructor(scope: Construct, id: string, props: QueueProps = {}) {
@@ -361,6 +366,7 @@ export class Queue extends QueueBase {
     this.queueName = this.getResourceNameAttribute(queue.attrQueueName);
     this.encryptionMasterKey = encryptionMasterKey;
     this.queueUrl = queue.ref;
+    this.deadLetterQueue = props.deadLetterQueue;
 
     function _determineEncryptionProps(this: Queue): { encryptionProps: EncryptionProps, encryptionMasterKey?: kms.IKey } {
       let encryption = props.encryption || QueueEncryption.UNENCRYPTED;

packages/@aws-cdk/aws-sqs/test/sqs.test.ts

@@ -31,7 +31,8 @@ test('default properties', () => {
 test('with a dead letter queue', () => {
   const stack = new Stack();
   const dlq = new sqs.Queue(stack, 'DLQ');
-  new sqs.Queue(stack, 'Queue', { deadLetterQueue: { queue: dlq, maxReceiveCount: 3 } });
+  const dlqProps = { queue: dlq, maxReceiveCount: 3 };
+  const queue = new sqs.Queue(stack, 'Queue', { deadLetterQueue: dlqProps });
 
   expect(stack).toMatchTemplate({
     'Resources': {
@@ -58,6 +59,8 @@ test('with a dead letter queue', () => {
       },
     },
   });
+
+  expect(queue.deadLetterQueue).toEqual(dlqProps);
 });
 
 test('message retention period must be between 1 minute to 14 days', () => {