Full protobuf support

[bernardnormier]

We should consider adding full Protobuf support, i.e.:

• add a protoc plugin that generates C# code for Protobuf services

I anticipate this code would be similar to the C# code generated for Slice interfaces, with client-side IName and NameProxy and server-side INameService.

• add an IceRpc.Protobuf.Tools assembly/package for the MSBuild integration and distribution of this native protoc plugin

• add examples and documentation (a new top-level Protobuf chapter next to Slice)

One open question is how to deal with failure responses for Protobuf service RPCs (status code > Success). We could for example mimic gRPC's "richer error model": https://grpc.io/docs/guides/error/

It relies heavily on Any, which is a concern for languages such as Rust: https://cloud.google.com/apis/design/errors#error_model

[pepone]

Seems rust already supports the richer error model:

• https://ericb.xyz/posts/rust-tonic-grpc-errors/
• https://github.com/hyperium/tonic/pull/1317

[pepone]

Implemented in #3697