Add new Auth APIs using AuthResult (#703)

Properly implement `AuthResult` and add the following methods which returns `Task<AuthResult>`
- `FirebaseAuth.SignInWithCustomTokenAsync()`
- `FirebaseAuth.SignInWithCredentialAsync()`
- `FirebaseAuth.SignInWithProviderAsync()`
- `FirebaseAuth.SignInAndRetrieveDataWithCredentialAsync()`
- `FirebaseAuth.SignInAnonymouslyAsync()`
- `FirebaseAuth.SignInWithEmailAndPasswordAsync()`
- `FirebaseAuth.CreateUserWithEmailAndPasswordAsync()`
- `FirebaseUser.ReauthenticateAndRetrieveDataAsync()`
- `FirebaseUser.ReauthenticateWithProviderAsync()`
- `FirebaseUser.LinkWithCredentialAsync()`
- `FirebaseUser.LinkWithProviderAsync()`
- `FirebaseUser.UnlinkAsync()`
- `FirebaseUser.UpdatePhoneNumberCredentialAsync()`

Misc
- Remove `SignInWithCredentialInternalAsync_DEPRECATED()` and rewire `SignInWithCredentialInternalAsync()` to the version returning `Future<User>` instead.

Author: chkuang-g

auth/src/FirebaseAccountLinkException.cs

@@ -26,11 +26,21 @@ public sealed class FirebaseAccountLinkException : System.Exception
 {
   /// Initializes a new FirebaseAccountLinkException, with the given error code and
   /// message and the AdditionalUserInfo returned from the Firebase auth service.
+  [System.Obsolete("Use `FirebaseAccountLinkException(int, string, AuthResult)` instead", false)]
   public FirebaseAccountLinkException(int errorCode, string message,
                                       SignInResult signInResult) : base(message)
   {
     ErrorCode = errorCode;
-    Result = signInResult;
+    result_DEPRECATED = signInResult;
+  }
+
+  /// Initializes a new FirebaseAccountLinkException, with the given error code and
+  /// message and the AdditionalUserInfo returned from the Firebase auth service.
+  public FirebaseAccountLinkException(int errorCode, string message,
+                                      AuthResult authResult) : base(message)
+  {
+    ErrorCode = errorCode;
+    result = authResult;
   }
 
   /// Returns the Auth defined non-zero error code.
@@ -43,10 +53,12 @@ public FirebaseAccountLinkException(int errorCode, string message,
   /// the credential may be used to sign-in the user into Firebase with
   /// Firebase.Auth.SignInWithCredentialAsync.
   public AdditionalUserInfo UserInfo {
-    get { return Result.Info; }
+    get { return (result != null) ? result.AdditionalUserInfoInternal :
+                 (result_DEPRECATED != null) ? result_DEPRECATED.Info : null; }
   }
 
-  private SignInResult Result { get; set; }
+  private SignInResult result_DEPRECATED = null;
+  private AuthResult result = null;
 }
 
 }