Add CocoaPods opt-ins for RN_STRICT_API and RN_ALLOW_FRAMEWORKS (#59037)

[coado]

Summary:

Let iOS apps that use CocoaPods enforce React Native's C++ stable API, and control its "for frameworks" warning, with two env vars read at pod install:

• RN_STRICT_API=1 defines the RN_STRICT_API macro, which arms the react/cxxstableapi guards. A direct include of a public module header is an error pointing to the module's umbrella (<React/<Module>.h>), a private header is an error, and a "for frameworks" header is a warning.
• RN_ALLOW_FRAMEWORKS=1 defines the RN_ALLOW_FRAMEWORKS macro, which silences that warning.

Each variable is named after the macro it defines, matching the Android opt-in, which passes RN_STRICT_API to CMake.

Both macros are added to the app project and the Pods project, so they reach the app, third-party pods and app codegen. React Native's own pods are unaffected because they define RN_BUILDING. Both are off by default; running pod install without a variable removes its define again. This mirrors the existing RCT_REMOVE_LEGACY_* opt-ins and the Android opt-in.

Who silences the "for frameworks" warning for which code:

• A library that uses for-frameworks headers declares RN_ALLOW_FRAMEWORKS for its own pod (pod_target_xcconfig). That covers its own build only and never reaches the app or other pods.
• The app author sets RN_ALLOW_FRAMEWORKS=1 before pod install for the app's own code. It also silences libraries that use those headers without declaring it, which is on the library.
• The generated ReactCodegen and ReactAppDependencyProvider pods always define RN_ALLOW_FRAMEWORKS: nobody can act on a warning in generated code, and libraries are already held to the API in their own pods.

Changelog:
[iOS][Added] - Add RN_STRICT_API and RN_ALLOW_FRAMEWORKS CocoaPods opt-ins to enforce React Native's C++ stable API in app and library native code

Reviewed By: cipolleschi

Differential Revision: D124979668

[meta-codesync]

@coado has exported this pull request. If you are a Meta employee, you can view the originating Diff in D124979668.