Improve RCT-Folly so that other dependencies don't have to specify the folly flags (#49969)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49969

Improved RCT-Folly podspec by configuring it as a prepare step, so that we don't need to forward the folly flags to all the others pods and to 3rd party dependencies.

## Changelog:

[INTERNAL] - Configure folly in a prepare step

Test Plan:
✅ Run Rn-Tester and  verify that it works as expected both building deps from source and using prebuilt tarballs
✅ Add third-party library (react-native-reanimated) and perform the same tests to verify that it works with the changed podspec and utilities

Reviewed By: javache

Differential Revision: D71032640

Pulled By: cipolleschi

fbshipit-source-id: a34a9b38fa5d5b37a0a193a73659f0d45dd3d660