Registry server: should PATCHing artifacts merge YAML fields?

[timburks]

As a followup to #1221, it seems useful to support some form of merging at the API level. Currently the Registry API only supports ReplaceArtifact, which differs from other resource types, because early API reviewers felt that artifacts should be atomic resources. This was prior to the emergence of a preference for YAML-formatted artifacts, which could be fairly easily merged. Should we revisit this in the API, perhaps with content type-specific features? (Maybe we only merge YAML and JSON artifacts)