Improve API types and documentation

Right now, our API routes (`/src/service/routes/*`) are undocumented and don't enforce strict request/response types causing confusion on how to use certain endpoints, especially for those that return `Action` objects or allow filtering responses.

This also prevents #1343 from being completed, since generating user-friendly documentation (in the form of a MDX doc) requires some preexisting API spec.

I thought about the best way to do this, and I like [tsoa](https://github.com/lukeautry/tsoa) for strengthening the types and also generating an OpenAPI reference which can be easily converted to MDX via another tool.

An alternative is to manually add the OpenAPI reference, but handling massive YAML docstrings in the endpoint code doesn't seem like such a good idea.

Any thoughts? @finos/git-proxy-maintainers

**Additional context**
#1343