Document roadmap / plans for cache server for buildstream 2.0

Background

The artifact cache server is a key component for a medium / large project.

Taking that into account, do we plan to consider it a first class citizen for 2.0 or we will rely on a external project to implement this functionality?

In the recent meetup, seems the following was concluded (https://docs.google.com/document/d/1bENn0gIHDPyTXaRwqSoA2F11gn55vvWLB7qMIe3Pk1U/edit#heading=h.9rzdt6usrgge):

• Agreed that BuildStream will continue to provide an artifact cache server
• Agreed that this will be in a separate repo to master
• Agreed that control and interrogation protos also to be moved into own repo
• New artifact server should be a reference implementation, based on buildbox
• CAS server functionality will either use BuildBox casd or BuildGrid's CAS server

Acceptance Criteria

• Plans exist in the form of wiki / issues / update documentation somewhere