Handle filter condition on create

[maennchen]

I implemented it so that the single create returns the existing row.

Any row that was skipped because of the upsert filter clause will have the metadata entry upsert_skipped set.

Alternatively we could return nil, but I believe that might confuse the users of the function. With bulk creates it makes more sense to be missing.

I couldn't find specific tests for single create / single bulk create. So I added those as well.

This is a prerequisite to implement the syntax sugar from https://github.com/ash-project/ash/issues/1385

Contributor checklist

• [X] Bug fixes include regression tests
• [ ] Features include unit/acceptance tests

[zachdaniel]

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