Make `react/renderer/imagemanager` public (#58438)

Summary:
Pull Request resolved: https://github.com/react/react-native/pull/58438

Classifies `react/renderer/imagemanager:imagemanager` as a public target under the C++ stable API three-tier visibility model. Replaces the `FrameworksGuard.h` include in the module's 7 exported headers with `<react/cxxstableapi/UmbrellaGuard.h>` and introduces the module umbrella `React/ImageManager.h`, wiring the umbrella into BUCK, CMake, CocoaPods, the iOS prebuild header config and the Android prefab export.

Changelog:
[General][Added] - Added umbrella header for the `react/renderer/imagemanager` module

Reviewed By: cortinico

Differential Revision: D119477737

fbshipit-source-id: 74eef15ee77f76de35f6ec98b20f6f9adf44f03d