Move source cache to proto based service

Background

The source cache should move towards using a protocol buffer based format similar to new artifact service architecture (overall plan described in #909 (closed)). The reference service should be kept for now to allow it to be used with older buildstream clients.

Task description

• Design new source protos. This should be similar to artifact protos and may include metadata such as the sources provenance data.
• Implement new SourceCacheService that uses this.
• Use new SourceCacheService in SourceCache for pulling and pushing sources.

Acceptance Criteria

All old source cache tests pass using new source protos.

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