Add support for per-message compression to Protobuf

[bernardnormier]

We encode Protobuf message using the gRPC framing, which includes a byte for compression.

We should add support for compression and decompression using that byte (bool) in the IceRpc.Protobuf encode/decode logic. Unlike what we do for Slice, I don't think we can use an IDL-independent interceptor and middleware. This needs to be combined with the Protobuf encode/decode logic.