Add Domain/Migration Rules and Metadata for Schema Registry

[andrewstucki]

Who is this for and what problem do they have today?

This is less of for a particular customer, and more of a "nice to have" to bring us closer to parity with Confluent's schema registry offering.

When doing complex schema evolution or enforcing additional validation rules for payload formats, schema registry allows for adding rules associated with a particular schema.

We support neither those nor metadata values.

What are the success criteria?

Allow for metadata and rule association with a schema.