Add the property `configuration` to `GIDGoogleUser` (#183)

* Add the property GIDConfiguration to GIDGoogleUser

- Add a GIDConfiguration to replace hostedDomain, serverClientID, openRealm.
- Remove the property serverAuthCode.

GoogleSignIn/Sources/GIDGoogleUser.m

@@ -173,6 +173,14 @@ NS_ASSUME_NONNULL_BEGIN
   return nil;
 }
 
+- (nullable NSString *)serverClientID {
+  return [_authState.lastTokenResponse.request.additionalParameters[kAudienceParameter] copy];
+}
+
+- (nullable NSString *)openIDRealm {
+  return [_authState.lastTokenResponse.request.additionalParameters[kOpenIDRealmParameter] copy];
+}
+
 #pragma mark - NSSecureCoding
 
 + (BOOL)supportsSecureCoding {