ESGF CV validation POC

[lliming]

Proof of concept for ESGF controlled vocabulary validation for publish, update, and replica add events.

• Document which schema fields are validated & where CVs are obtained
• Perform validation and throw errors if invalid
• Include validation info (e.g., CV versions) in events published on event stream

[sturoscy-personal]

Initial implementation is here using the esgvoc module.

This is likely to change as we are looking at implementing a STAC extension to take care of both the schema level and CV validation. More in other comments.