Support passing an already-initialized firebase app

[andrewrosss]

As the title suggests it would be nice to be able to just pass an already initialized firebase admin app to flask-firebase-admin.

At the moment flask-firebase-admin can be configured to use a previously-initialized app by setting

app = Flask(__name__)

# elsewhere ...
admin.initialize_app(...)

# then ...
app.config["FIREBASE_ADMIN_NAME"] = "name-of-your-already-initialized-admin-app"
app.config["FIREBASE_ADMIN_RAISE_IF_APP_EXISTS"] = False

firebase = FirebaseAdmin(app)

But this feels circuitous and something like

app = Flask(__name__)

# elsewhere ...
admin.initialize_app(...)

app.config["FIREBASE_ADMIN_APP"] = admin

firebase = FirebaseAdmin(app)

would probably be more natural.