client_live_key, client_test_key fixtures: provide client_id for correlation

[risicle]

Functioning similarly to driver_id and included in User-agent, this should make it possible to correlate requests from a particular FT process with its corresponding logs server-side.

[risicle]

A slight complication because get_notification_by_to_field uses its own api client instance which doesn't immediately seem to fit well into a fixture. Instead it's going to get its own client (and therefore client id) per call. But in many of these cases the stack trace helpfully contains the headers dict that shows the User-agent, meaning not having display-on-teardown isn't a complete loss.