Artifact as a Proto: protos and service

Background

Task 4 of #909 (closed). This will involve adding the protocol buffer files, and the grpc service to the bst-artifact-service. At the moment it will be separate from the rest of Buildstream, and so subject to later change, but should have the majority of work for this done. This will in future completely replace the reference service we currently use.

Task description

• Write protos
• Write grpc service:
  • GetArtifact
  • UpdateArtifact
• Write tests against grpc service

Acceptance Criteria

Passing tests that use the artifact service.

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