Add extra parameter to define whether codegen is invoked by lib or app (#48995)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48995

This change adds an extra parameter to the codegen script that allow our users to trigger codegen for Apps or for Libraries.

When running codegen for Apps, we have to generate some extra files that are not needed by the Libraries. This is causing issues to our library maintainers and this change will provide more flexibility in the DevX of libraries.

The default value is App, so if the new parameter is not passed, nothing will change in the current behavior.
[iOS][Added] - Add the `source` parameter to generate-codegen-artifacts to avoid generating files not needed by libraries.

Reviewed By: cortinico

Differential Revision: D68765478

fbshipit-source-id: 8030b4472ad4f5058e58b1c91089de5122a4f60a