can't disable timeouts in rpc calls

Most rpc calls support passing in a retry and timeout value to configure retry parameters (example: [WriteBatch.commit](https://github.com/googleapis/python-firestore/blob/f10b503b0dc2b23e43495e3f0417ae2630c13d82/google/cloud/firestore_v1/batch.py#L41))

But when `timeout=None`, that means it uses the api default, meaning there is no way to truly disable timeouts

That said, fixing this would likely be a breaking change, and timeout can always be set arbitrarily high, so this is a low-priority fix