Add example for to_bytes and trailers

[tiziano88]

It is not obvious how to deal with trailers, especially since in most cases the body gets consumed by a call to to_bytes, even though it is perfectly fine to call that on a mutable reference to the body, and then extract the trailers aftewards.

[tiziano88]

@mariaschett could you take a look?

[mariaschett]

Yes, I agree, we have needed trailers to proxy a gRPC request here.