FirebaseOptions to PigeonFirebaseOptions support

[TheFabbiusCorp]

With the latest firebase_core release, where Pigeon generation had been added, I'm finding it challenging to create a custom MockFirebaseApp without the support for converting from a pre-defined FirebaseOptions to the new PigeonFirebaseOptions. Due to this, I must manually create the PigeonFirebaseOptions using the default constructor even if I only want to copy the parameters from a given FirebaseOptions instance.