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/get_auth_url.dart

@@ -0,0 +1,43 @@
+import "dart:math";
+
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+part "get_auth_url.freezed.dart";
+part "get_auth_url.g.dart";
+
+@freezed
+abstract class OAuthGetAuthUrl with _$OAuthGetAuthUrl {
+  const factory OAuthGetAuthUrl({
+    required ResponseMode responseMode,
+    required Uri homeserverUrl,
+    required Uri redirectUri,
+    required IList<String> scopes,
+    required String clientId,
+    String? userIdHint,
+  }) = _OAuthGetAuthUrl;
+
+  factory OAuthGetAuthUrl.fromJson(Map<String, Object?> json) =>
+      _$OAuthGetAuthUrlFromJson(json);
+}
+
+abstract class Scope {
+  static final openid = "openid";
+  static final email = "email";
+  static final clientApi = "urn:matrix:client:api:*";
+
+  static final _deviceChars = IList(
+    ("ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+            "abcdefghijklmnopqrstuvwxyz"
+            "0123456789"
+            "-._~")
+        .split(""),
+  );
+
+  static String get _deviceId =>
+      _deviceChars.shuffle(Random.secure()).sublist(0, 10).join();
+
+  static String get device => "urn:matrix:client:device:$_deviceId";
+}
+
+@JsonEnum(fieldRename: .snake)
+enum ResponseMode { query, fragment }