4.0: Return a single record from Ash.read when the action has get?: true

[JohnDoneth]

Closes #1628

Make get?: true read actions cause Ash.read to return a single record, just like pagination causes it to return a page.

Contributor checklist

• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

[zachdaniel]

If we want to introduce this now, we have to gate it behind a compilation flag. Additionally this will almost certainly break other packages calling Ash.read. So I think we would be best suited waiting until 4.0 (that's why the 4.0 prefix in the issue name).