feat(http): allow RequestGuard to modify request

[rohansingh]

Update the signature of RequestGuard.Allowed to accept a pointer to the current Starlark thread, and to return an *http.Request.

This allows a RequestGuard implementation to modify the HTTP request before it is executed, and to do so on a per-thread basis if desired.

[rohansingh]

This is a breaking change on the RequestGuard interface, but it's also a small change that adds a lot of functionality. Use cases include:

• Redirecting certain requests to a cache.
• Rewriting or adding request headers so that it's clear which requests originate from Starlark.
• Substituting a new request with a custom http.RoundTripper. We use this internally to provide a whole set of in-process services to Starlark code via an HTTP interface.

[dustmop]

Sounds fine! Simple enough of a change, and it doesn't break our own usage in a major way. LGTM

[rohansingh]

Thanks for the quick merge! :)