Moved App Sources into separate files

lib/app_sources/signal.dart

@@ -0,0 +1,36 @@
+import 'dart:convert';
+import 'package:http/http.dart';
+import 'package:obtainium/providers/source_provider.dart';
+
+class Signal implements AppSource {
+  @override
+  late String host = 'signal.org';
+
+  @override
+  String standardizeURL(String url) {
+    return 'https://$host';
+  }
+
+  @override
+  Future<APKDetails> getLatestAPKDetails(String standardUrl) async {
+    Response res =
+        await get(Uri.parse('https://updates.$host/android/latest.json'));
+    if (res.statusCode == 200) {
+      var json = jsonDecode(res.body);
+      String? apkUrl = json['url'];
+      if (apkUrl == null) {
+        throw noAPKFound;
+      }
+      String? version = json['versionName'];
+      if (version == null) {
+        throw couldNotFindLatestVersion;
+      }
+      return APKDetails(version, [apkUrl]);
+    } else {
+      throw couldNotFindReleases;
+    }
+  }
+
+  @override
+  AppNames getAppNames(String standardUrl) => AppNames('Signal', 'Signal');
+}