[Feat]: Add support for streams pipelines

[sleipnir]

This closes #270

[sleipnir]

@polvalente, I think this is ready for review now. I would like to rename the modules:

GRPC.Server.Stream -> GRPC.Server.Materializer
and maybe their client-side equivalent.

Justification: I don't think they are a real Stream structure and in the new proposal they are only used to "materialize" the response to the client. It also looks a bit strange to have two modules named Stream in the library.
But I haven't gone ahead with this yet, waiting for your opinion.

The PR was a bit big but sorry there wasn't much that could be done since it's a likely breaking change.

[polvalente]

Sorry about the lengthy review. This overall is looking good, but seeing that we're aiming for this to mark v1.0, I think we can commit to the breaking change of merging the GRPC.Stream and GRPC.Server.Stream structs, and remove other kinds of streaming, even if we have to do this through multiple PRs.

[sleipnir]

@polvalente shall we continue?

[polvalente]

Just some minor comments and questions, but this looks mostly good to go (given the pending stuff we're leaving for follow-up PRs)

[sleipnir]

Just some minor comments and questions, but this looks mostly good to go (given the pending stuff we're leaving for follow-up PRs)

Great! I accepted all suggestions except two so we could discuss them further.