Expose unstable_fuseboxEnabled API on iOS (#44860)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44860

- Enables an opt-in to the Fusebox stack on iOS for both architectures in open source.
- Templates use of this opt-in in RNTester.

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D58364053

fbshipit-source-id: c604b1589174bf7cfd0fe1bfb5624c4edd0a125d