feat(types): Add gRPC Richer Error Model support

[flemosr]

Motivation

The gRPC richer error model is quite useful to send additional feedback to clients, and is supported by many gRPC libraries in other languages.

Solution

This PR adds richer error model support to tonic-types, as per #1034. Example servers and clients were also added.

[flemosr]

Note: there are links at the bottom of the added README.md that are supposed to point to the crate docs. They would have to be updated to point to the correct released version of tonic-types.

[LucioFranco]

Thank you for this! This is quite a large PR and will likely take me a while to review. Do you think you could reduce this into multiple PRs that we can merge independently?

[flemosr]

Hi! Happy to contribute. I think the new code can be more or less evenly splitted between the functionallity corresponding to each standard error message type (BadRequest, RetryInfo, etc). I could add only the functionallity corresponding to BadRequest on the first PR, and add support for the other error types in subsequent PR's. Would this approach work better?

[LucioFranco]

@flemosr yeah that would be great! Thanks

[flemosr]

Hi! The first PR is still quite big, but most of it is the new proto file, docs and tests. The following ones can be smaller