(amplify-alpha): The default created role is empty

### Describe the bug

When I create an amplify app, the associated role has no policies.

### Expected Behavior

If I do the same thing in the web interface it creates a role with the managed policy `AdministratorAccess-Amplify` attached

### Current Behavior

The role has no policies

### Reproduction Steps

```
const app = new amplify.App(this, 'WebApp', {
appName: 'web',
platform: amplify.Platform.WEB_COMPUTE,
sourceCodeProvider: new amplify.GitHubSourceCodeProvider({
owner,
repository,
oauthToken,
}),
});
```

### Possible Solution

I've worked around this by manually creating and passing in a role

### Additional Information/Context

_No response_

### CDK CLI Version

2.125.0

### Framework Version

_No response_

### Node.js Version

20.11.0

### OS

Linux

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

No source file or test is named. Start at the amplify.App construct and trace how its default role is created, then compare the generated role with the web interface behavior; done means the default role has the AdministratorAccess-Amplify managed policy attached while preserving the option to pass a custom role.

Written by the indexing model from the issue text.