Add Firebase App Check

[jjgriff93]

Add Firebase App Check package

This PR resolves #177 and adds App Check as a new package for use with Capacitor 4. This allows users to get App Check tokens from the native layer and use for device verification in their web layer, such as for use with the AngularFire library, Firebase Web SDK and others.

I've done my best to follow the semantics used in the other packages so that it fits with this library but please feel free to suggest/make any changes you feel appropriate.

Have tested with my app running on iOS and Android successfully but probably a good idea for you to test as well!

Any questions let me know, and consider this my thanks for putting together the other great packages in this repo! :)

Pull request checklist

Please check if your PR fulfills the following requirements:

• The changes have been tested successfully.
• A changeset has been created (npm run changeset).

[robingenz]

Hi @jjgriff93,
thank you for your contribution.
Unfortunately I can't accept this PR because you just copied the code from another GitHub project (https://github.com/mattmilan-dev/capacitor-firebase-appcheck) and don't even mention the original maintainer anywhere (or did I miss it?).

Apart from that, I prefer to create the boilerplate code of a new plugin myself (see comment) so that the review is not so time-consuming.

[jjgriff93]

@robingenz you're right, some code is taken from that repo but quite a lot is changed/updated, either way the maintainer does deserve a mention, my mistake. Feel free to use some of the logic or not in your own boilerplate when you get round to it.