Make response_body_filter async to support async operations

Hello,

Currently, the response_body_filter in Pingora is synchronous. For consistency with other phases and to allow executing asynchronous code inside the filter, it should also be made async.

This change would enable:

- Uniform async handling across all processing phases.
- Execution of async operations (e.g., I/O, DB calls) within response_body_filter.

It would improve both flexibility and consistency of the request/response pipeline.

Thank you for considering this improvement..