firestore: samples should include client creation #11993

Currently, we have one sample for creating a client, and the others assume the client already exists.

This causes problems, because we have both a sync and an async client, and using the wrong client leads to errors. When the samples are viewed stand-alone, they don't give the proper context

We should instead include the creation of the client (and any important imports) as part of each sample, so they can better stand alone