refactor: google, facebook, phone providers #1277
                
     Merged
            
            
          
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Hey there! So you want to contribute to FirebaseUI? Before you file this pull request, follow these steps:
withGoogleSignIn(scopes scopes: [String]? = nil)towithGoogleSignIn(_ provider: GoogleProviderSwift? = nil).withFacebookSignIn(scopes scopes: [String]? = nil)towithFacebookSignIn(_ provider: FacebookProviderSwift? = nil)so provider can be passed through.FacebookProviderAuthUItoFacebookProviderSwiftin keeping with other providers as per API doc and gave it the provider functionality tocreateAuthCredential()anddeleteUser()(needed as sensitive operation like delete user requires reauth).FacebookProviderAuthUInow renders the button and holdsFacebookProviderSwiftfor operations.GoogleProviderAuthUItoGoogleProviderSwiftin keeping with other providers as per API doc and gave it the provider functionality tocreateAuthCredential()anddeleteUser()(needed as sensitive operation like delete user requires reauth).GoogleProviderAuthUInow renders the button and holdsGoogleProviderSwiftfor operations.PhoneAuthProviderAuthUIProtocoltoPhoneAuthProviderSwiftin keeping with other providers as per API doc.PhoneAuthProviderAuthUItoPhoneProviderSwiftin keeping with other providers as per API doc.PhoneAuthProviderAuthUInow renders the button and holdsPhoneProviderSwiftfor operations.