[REST API + KafkaSQL] Significant updates to the REST API to change how artifact and version metadata behave

[EricWittmann]

• Artifacts no longer have state, only versions have state
• Removed the /state endpoints and moved state into the metadata - changing a version state is now done by updating the version metadata
• Artifact metadata is now its own thing, not inherited from the "latest" version metadata
• Artifacts no longer have content - only metadata and a collection of versions
• /groups/:groupId/artifacts/:artifactId now GETs and PUTs metadata, not content
• Removed the /groups/:groupId/artifacts/:artifactId/owner endpoint - owner is now just another metadata property

All of these changes required multiple storage layer updates which are relatively easy in SQL but annoying in KafkaSQL. As a result, I have changed the KafkaSQL layer to use more of a 1-1 RPC style approach rather than the existing approach that is based on log compaction. This means we are now committed to implementing snapshotting.

[apicurio-bot]

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