Provide `tonic::Request::get_timeout()`

[snprajwal]

Feature Request

We provide a set_timeout() method on tonic::Request, that sets the grpc-timeout header. It would naturally make sense to also provide a get_timeout() method, instead of requiring the user to do request.metadata().get("grpc-timeout") and processing the string into a std::time::Duration object manually.

Motivation

This just makes the API more consistent. I'm happy to open the PR if this will be accepted.