Change the CachedClient to support HTTP caching directives

The motivation is that retriable errors should always retry (rather than cache).

Even if they don't have `Cache-Control: no-store` directive, 5XX errors should also not be cached.

This is what we'd want to update: https://github.com/google/oss-rebuild/blob/5bde322b7e50338183856c1143ca0b5ed80a40ad/internal/httpx/http.go#L45-L73

See these mozilla docs: https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Caching

Read internal/httpx/http.go around lines 45-73 and consult the linked Mozilla caching guidance. Trace the CachedClient behavior for retriable errors, Cache-Control: no-store, and 5XX responses; done means those cases are not cached and retriable errors retry as requested.

Written by the indexing model from the issue text.