Allow out of tree platforms to provide customized saveAssets experiences

[acoates-ms]

Summary:

Currently the CLI provides logic to saveAssets for iOS/Android. -- with some combination of the two implementations working for the basic cases for other platforms. But as other platforms mature, they want to be able to provide more platform specific optimizations similar to how iOS provides logic to generate asset catalogs.

This change introduces a new property to react-native config for platform out of tree implementations to optionally provide. This property provides a location for a js module that can be used to call into a platform specific implementation of much of the saveAssets logic.

For example, macos would be able to duplicate the iOS asset catalog behavior. And windows could modify the asset filenames to align with its native scaling logic.

Test Plan:

microsoft/react-native-windows#11839 adds support for the new saveAssetsPlugin property to successfully relocate assets.

Checklist

• Documentation is up to date to reflect these changes.
• Follows commit message convention described in CONTRIBUTING.md

[szymonrybczak]

Hey @acoates-ms, since we migrated cli-plugin-metro package to React Native core, mind reopening this PR there? 🙏

[szymonrybczak]

Closing, see the comment above.