Caching: persist between runs

[woodruffw]

Right now we cache GitHub API responses between audits/workflows, but there are a lot of responses that also make sense to cache between zizmor runs (e.g. a commit that isn't an impostor isn't going to become an impostor).

[woodruffw]

Thinking about this more: I think almost all of the caching can be moved into HTTP middleware, since nothing else we do is really that expensive (or more precisely, so expensive that it makes sense to cache between runs).

For HTTP caching, this looks promising: https://github.com/06chaynes/http-cache/tree/main/http-cache-reqwest

[woodruffw]

#304 is an attempt at the above.