Added the HTML fallback Source

lib/app_sources/html.dart

@@ -0,0 +1,47 @@
+import 'package:easy_localization/easy_localization.dart';
+import 'package:html/parser.dart';
+import 'package:http/http.dart';
+import 'package:obtainium/custom_errors.dart';
+import 'package:obtainium/providers/source_provider.dart';
+
+class HTML extends AppSource {
+  @override
+  String standardizeURL(String url) {
+    return url;
+  }
+
+  @override
+  String? changeLogPageFromStandardUrl(String standardUrl) => null;
+
+  @override
+  Future<APKDetails> getLatestAPKDetails(
+    String standardUrl,
+    Map<String, dynamic> additionalSettings,
+  ) async {
+    var uri = Uri.parse(standardUrl);
+    Response res = await get(uri);
+    if (res.statusCode == 200) {
+      List<String> links = parse(res.body)
+          .querySelectorAll('a')
+          .map((element) => element.attributes['href'] ?? '')
+          .where((element) => element.toLowerCase().endsWith('.apk'))
+          .toList();
+      links.sort((a, b) => a.split('/').last.compareTo(b.split('/').last));
+      if (links.isEmpty) {
+        throw NoReleasesError();
+      }
+      var rel = links.last;
+      var apkName = rel.split('/').last;
+      var version = apkName.substring(0, apkName.length - 4);
+      List<String> apkUrls = [rel]
+          .map((e) => e.toLowerCase().startsWith('http://') ||
+                  e.toLowerCase().startsWith('https://')
+              ? e
+              : '${uri.origin}/$e')
+          .toList();
+      return APKDetails(version, apkUrls, AppNames(uri.host, tr('app')));
+    } else {
+      throw getObtainiumHttpError(res);
+    }
+  }
+}