[SR Support]: Add support for reading & writing key schemas from & to SR

[big-andy-coates]

1. Add support for reading and writing schemas to and from the schema registry.

• Pick up Integrate code paths for schema compatibility checks for AVRO and other formats #6133 as part of this work.
• Register key schema on CREATE and CREATE AS statements in the same way as is done for value schemas.
• Delete key subject on delete (and value subject for non-Avro, which may not currently be happening)
• Delete key subject on internal topics too when query terminated (and value subject for non-Avro, which may not currently be happening)

2. Doc updates

• Document the subject name for key schemas (and value if not already done) and when schemas are created / deleted

[agavra]

The above two PRs should get the bulk of this functionality done. We'll track follow-ups separately.