initial draft for feature parity change.

[Ughuuu]

First adding the id parameter to plugins so its easier to automate callback identifying.
Then creating some classes to match the interface of firebase.
The idea is to to do something like this with current firebase projects:

#if UNITY_WEBGL
import FirebaseWebglInterface
#else
import Firebase...
#endif

And have everything work as intented. If you don't want this change here, discard this PR.

This is still wip pr.