Add samples for Files API

[MarkDaoust]

First prototype of samples for python.

Ref: https://docs.google.com/spreadsheets/d/10X--DI0YPL3QfEMObe387rYg0t4-kuv33yKn2TIal7E/edit?resourcekey=0-kfy1Ry4YAZh6O4DltLjV9w&gid=0#gid=0

It's setup in a unittest framework, but this is intentionally not picked up by our presubmits yet.

We will need to:

1. Setup an API key for testing.
2. Decide if we want these to run presubmit, for everything or just when they're updated.