Corona to Solar2D Rebrand and Solar2D Plugin Directory

[Shchvova]

Hello
My name is Vlad and I am lead developer on Corona, and now Solar2D project. A official rebrand of the project. With it, we are moving away from the old marketplace, towards git based plugin distribution. I migrated most free plugins to it, including OneSignal.
We also had a release with AndroidX support and migration to new versions of Firebase. So I allowed myself to migrate your plugin to new system. Now it is GitHub based, all what is required is to push binaries to the directory.

I also migrated the android plugin to use gradle instead of jar. I had to use "delete" statement to remove placeholder, but otherwise plugin is much simpler, because all the metadata is imported from the gradle distributed aar.

If you want to have access to that repository, please let me know. I updated only android plugin, and tested it with a user.

Thank you for supporting Corona/Solar2D platform!