Feature request: add `_resolve_async()` and `_call_route_async()` for async resolver support

[leandrodamascena]

Use case

With AsyncMiddlewareFrame (PR 2) and _registered_api_adapter_async (PR 3) in place, we need to wire them into the actual resolve chain. This adds the async versions of _resolve() and _call_route() as private methods - the full async pipeline without exposing it to customers yet.

Solution/User Experience

• Add _resolve_async() - async version of _resolve(). Same route matching logic (sync, CPU-bound), but calls _call_route_async() instead of _call_route()
• Add _call_route_async() - builds async middleware chain using AsyncMiddlewareFrame with _registered_api_adapter_async as the final frame, then awaits the chain
• Add _build_middleware_stack_async() on Route - mirrors _build_middleware_stack() but returns an async chain
• Integration tests with async handlers going through the full chain
• Methods are private - not exposed to customers yet

Alternative solutions

Acknowledgment

• This feature request meets Powertools for AWS Lambda (Python) Tenets
• Should this be considered in other Powertools for AWS Lambda languages? i.e. Java, TypeScript, and .NET