Support Protobuf messages format

[igor-vovk]

Protobuf format is faster compared to JSON, but less convenient to work with (binary data in request and response, existing tools like Chrome's Web Developer Toolbar can not output it).
ConnectRPC can send and receive request/response messages in Protobuf format, when content-type is set to application/proto.
The initial support is already done, need to work on making sure conformance tests pass.

[igor-vovk]

Particular errors can be checked in conformance tests output https://github.com/igor-vovk/connect-rpc-scala/actions/workflows/scala.yml. Right now something is wrong with the way errors are serialized, needs to be figured out.