Publish generated proto clients in an idiomatic way

[znewman01]

When writing a client library for Sigstore, you'll need access to the protos. Manually copying them in is tedious and error-prone.

Instead, we should publish sigstore-protos libraries in a language-idiomatic way (e.g., on a popular package repository).

The languages that will need this:

• [ ] Ruby (RubyGems)
• [ ] Java (Maven Central) #33 #43 #45
• [ ] JavaScript (npm)
• [ ] Go? (Go just uses Git but I'm not sure how it works with generated files).
• [ ] Rust (Cargo)
• [ ] Python (PyPI) #20 #32 #34 #35

We'll probably want advice from representatives of each community, since no maintainer on this repo understands best practices for all of these languages unless somebody has weird hobbies they're not telling me about.