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/models/oauth_auth_code_response.dart

@@ -0,0 +1,15 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+part "oauth_auth_code_response.freezed.dart";
+part "oauth_auth_code_response.g.dart";
+
+@freezed
+abstract class OAuthAuthCodeResponse with _$OAuthAuthCodeResponse {
+  const factory OAuthAuthCodeResponse({
+    required String state,
+    required String codeVerifier,
+    required Uri url,
+  }) = _OAuthAuthCodeResponse;
+
+  factory OAuthAuthCodeResponse.fromJson(Map<String, Object?> json) =>
+      _$OAuthAuthCodeResponseFromJson(json);
+}