Implement the Remote Asset API

This adds initial support for the Remote Asset API for both local and proxy operation.

The proxy implementation populates the local cache on positive responses, however, it doesn't currently query the local cache before forwarding the request to the remote to make sure to never return a stale response. This allows clients that use dynamic instances to be in full control whether to query the local cache or the remote server by setting instance_name as appropriate.

Depends on https://gitlab.com/BuildGrid/buildbox/buildbox-common/-/merge_requests/306