feat(core): add support for `<optgroup>` tag

[nagaozen]

Reasons for making this change

HTML Select Element offers a native way to organize options in option groups. This update extends rjsf SelectWidget to support <optgroup> as well.

If this is related to existing tickets, include links to them as well. Use the syntax fixes #[issue number] (ex: fixes #123).

fixes #1813, #580

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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

[heath-freenome]

@nagaozen Thanks for the new feature... Can you update the CHANGELOG.md to add a new minor release with notes on this feature. Also please update the tests to validate these changes. Thanks

[nagaozen]

@nagaozen Thanks for the new feature... Can you update the CHANGELOG.md to add a new minor release with notes on this feature. Also please update the tests to validate these changes. Thanks

Sure, must update the CHANGELOG and place some tests.

[nagaozen]

Alright, checklist completed!

[heath-freenome]

@nagaozen Consider doing a npm run cs-format to fix your lint errors and then making sure using npm run lint

[heath-freenome]

@nagaozen Also, since the bootstrap-4 code has nearly the same implementation, can you also add this feature to it?

[nagaozen]

@nagaozen Also, since the bootstrap-4 code has nearly the same implementation, can you also add this feature to it?

Mmm, its a core widget, shouldn't it work as is in bootstrap 4? I've never played with the bs4 version, but will take a look into it.