OAuth Support (#53)

Rips out the old auth method, adds OAuth support. Fixes #51.

Tested Platforms:
- [x] Linux
    - [x] Flatpak
    - [x] Native (Nix)
- [x] Android
- [ ] Windows
- [ ] MacOS

Reviewed-on: Nexus/nexus#53

lib/controllers/auth_url.dart

@@ -0,0 +1,32 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/client.dart";
+import "package:nexus/controllers/client_id.dart";
+import "package:nexus/models/oauth_auth_code_response.dart";
+import "package:nexus/models/requests/oauth/get_auth_url.dart";
+
+class AuthUrlController extends AsyncNotifier<OAuthAuthCodeResponse> {
+  final Uri homeserver;
+  AuthUrlController(this.homeserver);
+
+  @override
+  Future<OAuthAuthCodeResponse> build() async => ref
+      .watch(ClientController.provider.notifier)
+      .getAuthUrl(
+        .new(
+          homeserverUrl: homeserver,
+          redirectUri: .new(scheme: "nexus.federated.nexus", path: "/"),
+          responseMode: .query,
+          scopes: .new([Scope.clientApi, Scope.device]),
+          clientId: await ref.watch(
+            ClientIdController.provider(homeserver).future,
+          ),
+        ),
+      );
+
+  static final provider =
+      AsyncNotifierProvider.family<
+        AuthUrlController,
+        OAuthAuthCodeResponse,
+        Uri
+      >(AuthUrlController.new);
+}