Fix 3759 - uneditable & permanent defaults with additional properties

[Piotr-Debicki]

Reasons for making this change

Fixes #3759

Additional properties with defaults set currently creates a permanent key-value pair that is essentially uneditable. Trying to edit the key creates an entirely new key value entry on blur while still keeping the default key-value pair. The default key-value pair is also not deletable. Solution here generates defaults for additional properties once on form initialization, and then skips regeneration on any form update

This solution works for root & nested additionalProperties (see tests) Allows for 'reset' form to regenerate additionalProperties defaults Also fixes the issue I mentioned in the Closed PR with custom formData not being applied when generating additionalProperties defaults

Checklist

• [x] I'm updating documentation
  • [x] I've checked the rendering of the Markdown text I've added
• [x] I'm adding or updating code
  • [x] I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • [x] I've updated docs if needed
  • [x] I've updated the changelog with a description of the PR
• [ ] I'm adding a new feature
  • [ ] I've updated the playground with an example use of the feature