Remote Execution CAS-based artifact cache

In line with the remote execution proposal, the plan is to introduce an artifact cache based on CAS as defined in the Remote Execution API. This can completely replace the existing tar and OSTree artifact cache backends with a single cross-platform implementation.

A complete solution for the server side is required. I.e., we need a CAS server that projects can install with suitable instructions without introducing unreasonable dependencies.

This will add grpcio as hard dependency and requires code generation for protocol buffers.

This depends on BuildStream no longer relying on OSTree summary files as there aren't any plans to support something similar with CAS on the server side, see #179 (comment 68766696).