Remove declaration of ResultSetMapping as sealed

Hibernate, and I am assuming other providers, have historical ways to define more rich ResultSet mappings.

Because ResultSetMapping was introduced as sealed and all of its impls as records, it is impossible to model those richer models using this API; which is a shame imo. Here is a PR to remove the seal and allow such usage.

A further step would be to make the impls NOT records. This would allow even better implementation of this idea by implementors. I held off on that for the time being until we had consensus on this idea in general.

Review the linked pull request for the proposed ResultSetMapping API change and inspect the current sealed declaration and its record implementations. The immediate scope is to allow provider-defined implementations by removing the seal; changing the implementations from records is explicitly deferred. Done means the API accepts richer provider-specific mappings without taking on that follow-up.

Written by the indexing model from the issue text.