Extract Codegen code from the react_native_pods to its own file (#34176)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/34176

It extracts the code related to the codegen from the main `react_native_pods` script to a dedicated file, adding also tests.

## Changelog

[iOS][Changed] - Move codegen in separate files

Reviewed By: cortinico

Differential Revision: D37755818

fbshipit-source-id: 99760d1def26ddbf065fdd234e0d183c2795513c