[camera][draft] Add camera plugin (#29)

Added the first draft version, but there are still a lot of unimplemented features.

packages/camera/example/elinux/flutter/main.dart

@@ -0,0 +1,12 @@
+//
+// Generated file. Do not edit.
+//
+// @dart=2.12
+
+import 'package:camera_example/main.dart' as entrypoint;
+import 'generated_plugin_registrant.dart';
+
+Future<void> main() async {
+  registerPlugins();
+  entrypoint.main();
+}