Proxy cache for testing

[Yohe-Am]

While on the nightly CI, we'll want to make sure any URLs are valid, for PR CI runs and local dev specially, using a proxy with caching can improve the test cycle by avoiding expensive downloads.

Deno has a proxy system in place that should be a great fit. Investigate env var based proxy enablement for curl and wget and other common http clients so that ports that use those programs to download will be covered as well.

Originally posted by @zifeo in #21 (comment)

[linear]

MET-507 Proxy cache for testing