Add support for deleting subject compatibility in schema registry

[weeco]

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

Redpanda's schema registry allows to register a subject compatibility level using the PUT /config/{subject} endpoint, but you can never unset and default to the global default again anymore, because the respective DELETE endpoint does not exist.

What are the success criteria?

The DELETE /config/{subject} endpoint is implemented in an API compatible fashion. The confluent schema registry PR can be found here: confluentinc/schema-registry#1780

Why is solving this problem impactful?

Without this endpoint you can never unset what you have set before.

Additional notes

[oleiman]

Question for @piyushredpanda and/or @weeco - are you (or anyone else) expecting/needing a backport of this feature?

[piyushredpanda]

I don't see a downside to backporting it, but then again, who is going to use it in prior releases (if not Console)? I suggest we not backport unless @weeco is going to use it with v23.2.x releases.

[weeco]

Console would indeed use this feature in v23.2. If not too much effort a backport would be nice I believe. Why? We are about to roll out the Console changes with a much better schema registry management. One of these new features is to edit the schema compatibility, but users wouldn't be able to unset a set compatibility mode.

I can't judge how problematic that would be for some of our users.

[oleiman]

If not too much effort a backport would be nice I believe

Looks like clean cherry-picks into 23.2.x and 23.1.x, so we should be all good 👍