feat: allow override form values with defaults

[Lonli-Lokli]

Reasons for making this change

Add the ability to override existing form values (especially arrays) with 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
• [x] I'm adding a new feature
  • [x] I've updated the playground with an example use of the feature