Support fetching CAS objects via the Bazel HTTP caching protocol (enables CDN-backed remote storage)

[jjardon]

BuildStream relies on buildbox-casd for all CAS access. The gRPC REAPI CAS protocol is efficient for low-latency environments but is difficult to put behind a standard CDN

Upstream work to add client-side support for this protocol as a remote in buildbox-casd's proxy mode is tracked here:
https://gitlab.com/BuildGrid/buildbox/buildbox/-/work_items/201