Add step to sign the XCFramework in CI (#49871)

Summary:
This PR adds a step in CI to sign the xcframework if we have a certificate available
bypass-github-export-checks
## Changelog:
[Internal] - Sign XCFrameworks in CI

Pull Request resolved: https://github.com/facebook/react-native/pull/49871

Test Plan: GHA

Reviewed By: cortinico

Differential Revision: D70734340

Pulled By: cipolleschi

fbshipit-source-id: 244422bdf9a8e0a2075a727a91805f762d4f95c9