feat(apigatewayv2): add description property for stage (#30820)

### Issue # (if applicable)

### Reason for this change

The `description` property existed in the L1 construct but was not present in the L2 construct

### Description of changes

Add the `description`  property for `HttpStage` and `WebSocketStage`, which was missing in the L2 construct.

### Description of how you validated changes

I Added both unit test and integration 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*

Author: yoyoyo-pg

packages/@aws-cdk-testing/framework-integ/test/aws-apigatewayv2/test/http/integ.stage.js.snapshot/aws-cdk-aws-apigatewayv2-http-stage.assets.json

@@ -13,11 +13,12 @@
     "ApiId": {
      "Ref": "HttpApiF5A9A8A7"
     },
-    "StageName": "$default",
     "DefaultRouteSettings": {
      "ThrottlingBurstLimit": 1000,
      "ThrottlingRateLimit": 1000
-    }
+    },
+    "Description": "My Stage",
+    "StageName": "$default"
    }
   }
  },

packages/@aws-cdk-testing/framework-integ/test/aws-apigatewayv2/test/http/integ.stage.js.snapshot/aws-cdk-aws-apigatewayv2-http-stage.template.json

@@ -12,6 +12,7 @@ new apigw.HttpStage(stack, 'HttpStageWithProperties', {
     rateLimit: 1000,
     burstLimit: 1000,
   },
+  description: 'My Stage',
 });
 
 app.synth();

packages/@aws-cdk-testing/framework-integ/test/aws-apigatewayv2/test/http/integ.stage.js.snapshot/cdk.out

@@ -14,11 +14,12 @@
     "ApiId": {
      "Ref": "WebSocketApi34BCF99B"
     },
-    "StageName": "dev",
     "DefaultRouteSettings": {
      "ThrottlingBurstLimit": 1000,
      "ThrottlingRateLimit": 1000
-    }
+    },
+    "Description": "My Stage",
+    "StageName": "dev"
    }
   }
  },

packages/@aws-cdk-testing/framework-integ/test/aws-apigatewayv2/test/http/integ.stage.js.snapshot/integ.json

@@ -13,6 +13,7 @@ new apigw.WebSocketStage(stack, 'WebSocketStage', {
     rateLimit: 1000,
     burstLimit: 1000,
   },
+  description: 'My Stage',
 });
 
 app.synth();