AuthorizationServiceConfiguration.fromUrl support

Nobody has claimed this yet.

I am building a AppAuth service similar to #407 and i would like to have the option to use the fetchFromUrl() with the discovery url to load my service config as part of the service initialization, before atempting to log in with the service.

I saw the Android AppAuth library offers a fetchFromUrl function for the
[AuthorizationServiceConfiguration](https://github.com/openid/AppAuth-Android/blob/master/library/java/net/openid/appauth/AuthorizationServiceConfiguration.java) so maybe there could be option to add it here as well.

````
final FlutterAppAuth _appAuth = FlutterAppAuth();
Completer oidcInitCompleter = Completer();
late final String _clientId;
late final String _discoveryUrl;
late final String _redirectUrl;
AuthorizationServiceConfiguration? _serviceConfiguration;

AppAuthService() {
_discoveryUrl = someDiscoveryURI;
_clientId = someClientID;
_redirectUrl = "someURI;
/* Wanted Feature:
_serviceConfiguration = await _appAuth.fetchFromUrl(_discoveryUrl);
*/
oidcInitCompleter.future.then((value) => notifyListeners())
}
````

Start by comparing the Android AuthorizationServiceConfiguration.fetchFromUrl implementation linked in the issue and reviewing the existing service-initialization flow related to #407. Determine how the Flutter API should expose discovery-based configuration before login, then verify that the behavior works across the Android and iOS plugin implementations.

Written by the indexing model from the issue text.