Add support for multiple Firebase projects, and switching between apps for Firebase Authentication

[emmernme]

This PR implements support for initializing multiple Firebase Apps with custom configurations. This can be used by individual plugins by overriding the currently used app. For now, this is only implemented and tested for Firebase Authentication, as this was the use case I personally needed, but it should be simple enough to implement for other plugins as well.

I have not had the change to thoroughly test the web implementation of this, so that may need some more work.

Re. issue #653.

Pull request checklist

• [On iOS and Android] The changes have been tested successfully.
• [Y] A changeset has been created (npm run changeset).
• [Y] I have read and followed the pull request guidelines.

[emmernme]

I've made the desired changes, and implemented FirebaseApp.getApps() so that it returns an array of app names – we could adapt this to return more info, such as the appId, but that doesn't seem necessary as the usage of initializeApp ensures the user can keep track of app names and the connected options in their implementation.

[robingenz]

Thanks, i will get back to you soon. I have a lot to do at the moment.

[emmernme]

Just letting you know, I've discovered several bugs in the Android implementation that will need fixing before this can be implemented. I'm currently pretty stuck, but will update once I've come up with a solution.