add method to updates User email (#132)

Author: Luizfelipefm

src/Firebase.Auth/FirebaseAuthProvider.cs

@@ -199,7 +199,19 @@ public async Task<FirebaseAuthLink> ChangeUserPassword(string idToken, string pa
             return await this.ExecuteWithPostContentAsync(GoogleUpdateUserPassword, content).ConfigureAwait(false);
         }
 
+        /// <summary>
+        /// Change a email from an user with his token.
+        /// </summary>
+        /// <param name="idToken"> The Token from an user. </param>
+        /// <param name="newEmail"> The new email. </param>
+        /// <returns> The <see cref="FirebaseAuth"/>. </returns>
+        public async Task<FirebaseAuthLink> ChangeUserEmail(string idToken, string newEmail)
+        { 
+            var content = $"{{\"idToken\":\"{idToken}\",\"email\":\"{newEmail}\",\"returnSecureToken\":true}}";
 
+            return await this.ExecuteWithPostContentAsync(GoogleUpdateUserPassword, content).ConfigureAwait(false);
+        }
+        
         /// <summary>
         /// Creates new user with given credentials.
         /// </summary>

src/Firebase.Auth/IFirebaseAuthProvider.cs

@@ -54,6 +54,14 @@ public interface IFirebaseAuthProvider
         /// <returns> The <see cref="FirebaseAuth"/>. </returns>
         Task<FirebaseAuthLink> SignInWithCustomTokenAsync(string customToken);
 
+        /// <summary>
+        /// Updates the User email.
+        /// </summary>
+        /// <param name="idToken"> The idToken of an authenticated user. </param>
+        /// <param name="newEmail"> The new email</param>
+        /// <returns></returns>
+        Task<FirebaseAuthLink> ChangeUserEmail(string idToken, string newEmail);
+
         /// <summary>
         /// Updates profile (displayName and photoUrl) of user tied to given user token.
         /// </summary>