feat(amplify): enables apps hosted with server side rendering (#26861)

### Problem:
The Amplify App L2 construct does not support the ability for customers to configure a SSR app without reaching down into the lower level L1.

### Solution:
Expose the `platform` field used to set SSG vs SSR hosted applications. Do not expose `WEB_DYNAMIC` as SSRv1 is on the deprecation path.

### Testing Done:
* `yarn build+test`
* added new tests for `platform` field
* updated snapshots for new default value

Closes #24076 and #23325

----

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

Author: srquinn21

Diff for: packages/@aws-cdk/aws-amplify-alpha/README.md

@@ -225,6 +225,16 @@ const amplifyApp = new amplify.App(this, 'App', {
 });
 ```
 
+## Configure server side rendering when hosting app
+
+Setting the `platform` field on the Amplify `App` construct can be used to control whether the app will host only static assets or server side rendered assets in addition to static. By default, the value is set to `WEB` (static only), however, server side rendering can be turned on by setting to `WEB_COMPUTE` as follows:
+
+```ts
+const amplifyApp = new amplify.App(this, 'MyApp', {
+  platform: amplify.Platform.WEB_COMPUTE,
+});
+```
+
 ## Deploying Assets
 
 `sourceCodeProvider` is optional; when this is not specified the Amplify app can be deployed to using `.zip` packages. The `asset` property can be used to deploy S3 assets to Amplify as part of the CDK:

Diff for: packages/@aws-cdk/aws-amplify-alpha/lib/app.ts

@@ -158,6 +158,15 @@ export interface AppProps {
    * @default - a new role is created
    */
   readonly role?: iam.IRole;
+
+  /**
+   * Indicates the hosting platform to use. Set to WEB for static site
+   * generated (SSG) apps (i.e. a Create React App or Gatsby) and WEB_COMPUTE
+   * for server side rendered (SSR) apps (i.e. NextJS).
+   *
+   * @default - WEB
+   */
+  readonly platform?: Platform;
 }
 
 /**
@@ -248,6 +257,7 @@ export class App extends Resource implements IApp, iam.IGrantable {
       oauthToken: sourceCodeProviderOptions?.oauthToken?.unsafeUnwrap(), // Safe usage
       repository: sourceCodeProviderOptions?.repository,
       customHeaders: props.customResponseHeaders ? renderCustomResponseHeaders(props.customResponseHeaders) : undefined,
+      platform: props.platform || Platform.WEB,
     });
 
     this.appId = app.attrAppId;
@@ -528,3 +538,16 @@ function renderCustomResponseHeaders(customHeaders: CustomResponseHeader[]): str
 
   return `${yaml.join('\n')}\n`;
 }
+
+export enum Platform {
+  /**
+   * WEB - Used to indicate that the app is hosted using only static assets.
+   */
+  WEB = 'WEB',
+
+  /**
+   * WEB_COMPUTE - Used to indicate the app is hosted using a combination of
+   * server side rendered and static assets.
+   */
+  WEB_COMPUTE = 'WEB_COMPUTE',
+}

Diff for: packages/@aws-cdk/aws-amplify-alpha/test/app.test.ts

@@ -442,3 +442,25 @@ test('with custom headers', () => {
     },
   });
 });
+
+test('create a statically hosted app by default', () => {
+  // WHEN
+  new amplify.App(stack, 'App', {});
+
+  // THEN
+  Template.fromStack(stack).hasResourceProperties('AWS::Amplify::App', {
+    Platform: amplify.Platform.WEB,
+  });
+});
+
+test('create a dynamically rendered app when the platform is set to WEB_COMPUTE', () => {
+  // WHEN
+  new amplify.App(stack, 'App', {
+    platform: amplify.Platform.WEB_COMPUTE,
+  });
+
+  // THEN
+  Template.fromStack(stack).hasResourceProperties('AWS::Amplify::App', {
+    Platform: amplify.Platform.WEB_COMPUTE,
+  });
+});

Diff for: packages/@aws-cdk/aws-amplify-alpha/test/integ.app-asset-deployment.js.snapshot/cdk-amplify-app-asset-deployment.template.json

@@ -29,7 +29,8 @@
       "AppRole1AF9B530",
       "Arn"
      ]
-    }
+    },
+    "Platform": "WEB"
    }
   }
  },
@@ -67,4 +68,4 @@
    ]
   }
  }
-}
+}

Diff for: packages/@aws-cdk/aws-amplify-alpha/test/integ.app-codecommit.js.snapshot/cdk-amplify-codecommit-app.template.json

@@ -67,7 +67,8 @@
       "Repo02AC86CF",
       "CloneUrlHttp"
      ]
-    }
+    },
+    "Platform": "WEB"
    }
   },
   "AppmainF505BAED": {
@@ -119,4 +120,4 @@
    ]
   }
  }
-}
+}

Diff for: packages/@aws-cdk/aws-amplify-alpha/test/integ.app.js.snapshot/cdk-amplify-app.template.json

@@ -79,7 +79,8 @@
       "AppRole1AF9B530",
       "Arn"
      ]
-    }
+    },
+    "Platform": "WEB_COMPUTE"
    }
   },
   "AppmainF505BAED": {
@@ -137,4 +138,4 @@
    ]
   }
  }
-}
+}

Diff for: packages/@aws-cdk/aws-amplify-alpha/test/integ.app.ts

@@ -25,6 +25,7 @@ class TestStack extends Stack {
           },
         },
       ],
+      platform: amplify.Platform.WEB_COMPUTE,
     });
 
     amplifyApp.addCustomRule({