Added Firestore's equivalent to RDB's FirebaseList and AnimatedFirebaseList - #757

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Had to add ? to DocumentSnapshot's documentID getter, it was returning an exception on the tests.

/// Returns the ID of the snapshot's document
String get documentID => _path?.split('/')?.last;

Also, removed new keywords from the example and changed MessageList to the new FirestoreAnimatedList.

Is this ready for review as the checks say or should I add anything else, @kroikie?

Edit: also, if anyone wants to test this before merging, just add this to your pubspec:

cloud_firestore:
  git:
    url: [email protected]:flutter/plugins.git
    path: packages/cloud_firestore
    ref: f380311

(if git@ doesn't work, change it to https://)

@gildaswise Thanks for this PR, however we are trying to focus on getting these plugins functionally complete before adding UI widgets. We would also likely make those UI widgets in a separate repo. I'll close this one, look out for when we start work on the UI widgets.

@kroikie Would be okay if I released this as a separate Dart plugin depending on cloud_firestore?

@gildaswise Sure, that would be cool however note that the cloud_firestore plugin is not yet 1.0.0 so it may change so if you are ok with a bit of change then go for it.

@kroikie Would be okay if I released this as a separate Dart plugin depending on cloud_firestore?

Can you please mention here name of this plugin after release?

@Elgeor No problem! It'll be firestore_ui and I'll release it in a few hours

I actually got a lot busy and more than a few hours have passed, but finally firestore_ui is available as a package! You can see it on pub here!

@Elgeor @branflake2267 @kroikie