chore(amplify-alpha): backfill missing enums for amplify-alpha (#33677)

### Description of changes

chore(amplify-alpha): backfill missing enums for amplify-alpha

### 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-amplify-alpha/lib/app.ts

@@ -569,6 +569,11 @@ export enum Platform {
    * server side rendered and static assets.
    */
   WEB_COMPUTE = 'WEB_COMPUTE',
+
+  /**
+   * WEB_DYNAMIC - Used to indicate the app is hosted using a fully dynamic architecture, where requests are processed at runtime by backend compute services.
+   */
+  WEB_DYNAMIC = 'WEB_DYNAMIC',
 }
 
 /**