fix(build): Correctly convert Empty to ()

[tdyas]

Motivation

#521 covers the work for Tonic to compile the well-known protobuf types instead of relying on the prost-types crate. (One reason to do that is for projects that want google.protobuf.Any to use Bytes instead of Vec<u8> for binary data.)

#522 added initial support for Tonic to configure Prost and itself to compile the well-known protobuf types.

That support is incomplete, however: tonic-build fails to compile when configured to convert google.protobuf.Empty to (). This configuration occurs if the user calls config.extern_path(".google.protobuf.Empty", "()") where config is an instance of prost_build::Config. The error occurs because the existing code tries to interpret () as a Rust path and parsing, for example, super::() as a Rust path will always fail.

Solution

This PR fixes the conversion of google.protobuf.Empty to () by specifically allowing the use of () as a valid Rust conversion without attempting to prepend any Rust path.

There are several stacked commits in this PR:

1. Update the wellknown-compiled integration test to configure the google.protobuf.Empty -> () conversion. This first commit fails to compile without the fix.
2. Refactor tonic_build::prost::Service::request_response_name to remove the duplication of logic between converting request and response types.
3. Fix the conversion by introducing an allow list of types (consisting only of () for now) and just generate a token stream in that case without trying to interpret as a Rust path.

[LucioFranco]

@tdyas if you can rebase this against master I think we can get this merged soon, thanks.

[tdyas]

Rebased cleanly and pushed.

[tdyas]

@LucioFranco: Tests pass after rebase; this should be ready to go.

[LucioFranco]

Thanks!