Convert strings passed for an in query on a DocumentID to DocumentRefs for ease of use

[crwilcox]

When querying a collection with an in filter on a documentId, passing an array of strings will not work, instead returning an error

Repro:

from google.cloud import firestore

client = firestore.Client("your-project-id")
collection = client.collection('0')

query = collection.where("__name__", "in", ["hello", "world"])
[i for i in query.stream()]

Node.js handles this today by validating the reference and as part of it doing a conversion. It would be convenient for go users if a similar conversion were done.
Referenced Node Code: https://github.com/googleapis/nodejs-firestore/blob/9a1a9dcd56dc125159be3ce7356a6ebce04519a9/dev/src/reference.ts#L1646

This code works today, so this is a matter of detecting the type mismatch with DocumentID queries and adjusting it on the user's behalf.

from google.cloud import firestore

client = firestore.Client("crwilcox-firestore")
collection = client.collection('0')

docRefs = [
    collection.document("hello"),
    collection.document("world")
]
query = collection.where("__name__", "in", docRefs)
[i for i in query.stream()]

Related Go Issue: googleapis/google-cloud-go#4151