Add FirestoreBuilder widget as a replacement to StreamBuilder

[rrousselGit]

As the moment, FIrestore is typically used like do:

StreamBuilder<DocumentSnapshot>(
  stream: FirebaseFirestore.instance.collection('movies').doc(id).snapshots(),
  builder: (context, AsyncSnapshot<DocumentSnapshot> snapshot) { ...}
)

The problem with this approach is, whenever the widget that created the StreamBuilder is rebuilt, a new Stream is created. Similarly, if we have two StreamBuilder rebuilding to the same reference, that will still create two streams and two connections with the native. Both of which are not ideal for performance

We could solve this problem by introducing a FirestoreBuilder, which would work like StreamBuilder, but instead of taking a Stream as parameter, it would take a document/collection/query reference:

FirestoreBuilder<DocumentSnapshot>(
  ref: FirebaseFirestore.instance.collection('movies').doc(id),
  builder: (context, AsyncSnapshot<DocumentSnapshot> snapshot) { ...}
)

Since references are immutable, this would enable FirestoreBuilder to not recreate the snapshot stream when the widget rebuilds, while also potentially caching the stream across all FirestoreBuilder instances (if desired at all).

Some open-ended questions:

• what if users want to do things like .snapshots().where or .map?
  This is potentially not a problem since queries support filter operations and the recently added withConverter should solve map use-cases.