[Auth] Add phone factor ID to the PhoneMultiFactorGenerator object (#5487)

* Add phone factor ID to the PhoneMultiFactorGenerator object

* Api review

* Add changeset

Author: sam-gc

.changeset/neat-pants-wonder.md

@@ -0,0 +1,5 @@
+---
+"@firebase/auth": patch
+---
+
+Add missing phone FACTOR_ID static property to the PhoneMultiFactorGenerator class

common/api-review/auth.api.md

@@ -596,6 +596,7 @@ export interface PhoneMultiFactorEnrollInfoOptions {
 // @public
 export class PhoneMultiFactorGenerator {
     static assertion(credential: PhoneAuthCredential): PhoneMultiFactorAssertion;
+    static FACTOR_ID: string;
 }
 
 // @public

packages/auth/src/platform_browser/mfa/assertions/phone.ts

@@ -90,4 +90,9 @@ export class PhoneMultiFactorGenerator {
   static assertion(credential: PhoneAuthCredential): PhoneMultiFactorAssertion {
     return PhoneMultiFactorAssertionImpl._fromCredential(credential);
   }
+
+  /**
+   * The identifier of the phone second factor: `phone`.
+   */
+  static FACTOR_ID = 'phone';
 }