Make C++ requirement opt-in (#27730)

Summary:
When building as a framework these headers get automatically added to the framework umbrella header for React-Core. Instead of converting all the React sources to ObjC++ files and still forcing external users that build native source (and link against a framework build) to also compile as ObjC++, this makes the attribution related methods that were added in https://github.com/facebook/react-native/commit/fdcdca4 opt-in to ObjC++ builds.

This is also the reason for the current failure of the CI `test_ios_frameworks` run.

## Changelog

I’m unsure if this change really warrants an entry in the CHANGELOG, as it’s more of an amendment of the (afaik) unreleased [change](https://github.com/facebook/react-native/commit/fdcdca4).

[iOS] [Fixed] - Make framework builds work again by making `RCTImageLoader` C++ requirement opt-in
Pull Request resolved: https://github.com/facebook/react-native/pull/27730

Test Plan:
I tested static and dynamic (framework) builds and ran the test suite.

This change should make the `test_ios_frameworks` CI run _build_ again, ~~but it may still fail overall as in my local testing one of the tests leads to a segfault (which I will try to address separately)~~.

Reviewed By: PeteTheHeat

Differential Revision: D19348846

Pulled By: fkgozali

fbshipit-source-id: 8a74e6f7ad3ddce2cf10b080b9a5d7b399bd5fc0