Add Auth Blocking samples for v1 & v2 functions

[colerogers]

• Creates a beforeUserCreated & beforeUserSignedIn function for v2
• Creates a beforeCreate & beforeSignIn function for v1
• Blocks account creation if the email is not a 'gmail' domain
• Blocks sign in if the email is part of the banned list in Firestore

[jhuleatt]

@kevinthecheung and @rachelmyers, I'd like your LGTM before merging - can you please give this a review?