Fast-load images from remote pull service

[jacobwgillespie]

WIP, but this replaces the local registry proxy container with pulling from a remote service (https://github.com/depot/pull.depot.dev). This simplifies the local CLI machinery, so that we don't have to run the local proxy container.

It works like this:

1. We'd run the pull.depot.dev service in AWS - this runs the same code that the registry proxy would have run
2. The CLI encodes the BuildKit connection details (address, TLS certs) into the registry auth password and pulls from the remote registry

I haven't deployed pull.depot.dev to AWS - I think we'd also want the hostname of the pull service to be provided by the API, so that we can modify which service the CLI pulls from based on region or self-hosted.