WIP: Add json (serde) support

[cynecx]

I started this work because I'm also working on adding Connect RPC support to tonic. So having proper protobuf json support is the first step into achieving that goal.

This (unfortunately very big) PR adds json/serde support to prost. This is still a WIP effort but it is almost feature complete and passes all conformance tests except tests related to fieldmasks.

A non-exhaustive list of TODOs:

• Decide whether the whole "custom serde" design is the one we want to persue
• Properly document code and public api
• Properly put this all behind a feature flag
• Probably split things up because this PR also includes a new Any implementation
• ...

So I am just putting this up here to get a general vibe check...

[cynecx]

However this PR is way too big to review. Can you split this work into smaller PRs?

Yeah, it's actually on the TODO list. I just still haven't gotten to actually complete this as a whole yet.