[REST API v3] Change how create-artifact and create-version APIs work!

[EricWittmann]

Overview

The following changes are included in this PR:

1. Create Artifact REST API changes
2. Create Version REST API changes
3. Content-Type now required for content operations
4. Content-Type is now preserved (if you add YAML content it will be stored as YAML, not JSON)
5. Removed stale CNCF compat API
6. Removed old Event Sourcing support (will be replaced with outbox pattern later)
7. Simplified Storage interface for creating artifacts and versions

Create Artifact REST API changes

• Empty artifacts can now be created (no versions).
• Removed the "create with version content" variant
• All calls now only accept application/json content (of CreateArtifact type)
• If an initial version is created, content-type is required
• All metadata formerly included in HTTP headers is now passed in the JSON body

Create Version REST API changes

• All calls now only accept application/json content (of CreateVersion type)
• All metadata is passed as part of the CreateVersion body
• Content-Type is required

[apicurio-bot[bot]]

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