[Gen 2] Support cross-region functions

### Describe the feature you'd like to request

similar to gen 1: https://docs.amplify.aws/gen1/react/build-a-backend/graphqlapi/custom-business-logic/#calling-functions-in-different-regions
reference a existing function in gen 2 can be done using `a.handler.function('')`
but cross region isnt supported

### Describe the solution you'd like

`a.handler.function('', '')`

### Describe alternatives you've considered

override backend.ts to addLambdaDataSource

### Additional context

_No response_

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request

### Would this feature include a breaking change?

- [ ] ⚠️ This feature might incur a breaking change

Start by tracing the Gen 2 `a.handler.function` entry point and the `backend.ts` override path that uses `addLambdaDataSource`. Compare the requested behavior with the linked Gen 1 cross-region function documentation. Done means an existing function can be referenced with both its name and region without requiring the override.

Written by the indexing model from the issue text.