[FR]:   FIRMessaging class to include a messagingForApp() to allow for handling multiple configurations.

### Description

Summary
Extend the FIRMessaging class to include a messagingForApp() method similar to the FIRDatabase's databaseForApp() method to allow for handling multiple configurations seamlessly.

### API Proposal

let myApp = FirebaseApp.app(name: "secondary_app")!
let messaging = Messaging.messaging(app: myApp)


### Firebase Product(s)

Messaging