rn_codegen: define a method to convert package.json codegenConfig into Buck format (#36868)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/36868

Given a codegenConfig definition in a library's package.json, convert it into Buck specific codegen format, as defined by the kwargs of rn_codegen().

Reference:
https://reactnative.dev/docs/next/new-architecture-library-intro#configure-codegen

Changelog: [Internal]

Reviewed By: cortinico, mdvacca

Differential Revision: D44857743

fbshipit-source-id: 82011161d43cebe701cd2b129208054c21d3175a