chore(amplify-alpha): migrate asset deployment handler (#27955)

This PR moves the migrate asset deployment handler to our new centralized location for custom resource handlers in the [@aws-cdk](https://github.com/aws-cdk) package.

----

*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/branch.ts

@@ -233,10 +233,7 @@ class AmplifyAssetDeploymentProvider extends NestedStack {
       this,
       'amplify-asset-deployment-on-event',
       {
-        entry: path.join(
-          __dirname,
-          'asset-deployment-handler/index.js',
-        ),
+        entry: path.join(__dirname, '..', '..', 'custom-resource-handlers', 'dist', 'aws-amplify-alpha', 'asset-deployment-handler', 'index.js'),
         handler: 'onEvent',
         initialPolicy: [
           new iam.PolicyStatement({
@@ -257,10 +254,7 @@ class AmplifyAssetDeploymentProvider extends NestedStack {
       this,
       'amplify-asset-deployment-is-complete',
       {
-        entry: path.join(
-          __dirname,
-          'asset-deployment-handler/index.js',
-        ),
+        entry: path.join(__dirname, '..', '..', 'custom-resource-handlers', 'dist', 'aws-amplify-alpha', 'asset-deployment-handler', 'index.js'),
         handler: 'isComplete',
         initialPolicy: [
           new iam.PolicyStatement({