Add a py.typed file for submodule

[crwilcox]

To be in accordance with PEP 561 we should add a py.typed file to google/cloud/datastore/py.typed

The file can be empty but does need to be present for some type checkers.

Also, for this to work, google/cloud/spanner.py needs to be restructured to google/cloud/spanner/init.py

See googleapis/python-firestore#447 for additional context

[crwilcox]

Do note, you may need to run mypy, pyright, pytype against our samples before merging. It will ensure that we aren't exposing users to failing type checks.

Ideally we would add this to CI as well, to avoid future breakage.