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/requests/oauth/exchange_token.dart

@@ -0,0 +1,17 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+part "exchange_token.freezed.dart";
+part "exchange_token.g.dart";
+
+@freezed
+abstract class OAuthExchangeTokenRequest with _$OAuthExchangeTokenRequest {
+  const factory OAuthExchangeTokenRequest({
+    required Uri homeserverUrl,
+    required String codeVerifier,
+    required Uri redirectUri,
+    required String code,
+    required String clientId,
+  }) = _OAuthExchangeTokenRequest;
+
+  factory OAuthExchangeTokenRequest.fromJson(Map<String, Object?> json) =>
+      _$OAuthExchangeTokenRequestFromJson(json);
+}