[batch] Have Batch Worker perform auth checks on requests from Batch

[daniel-goldstein]

We should already have measures in place (like firewall rules) that prevent untrusted code from reaching the Batch Worker server, but this provides an extra layer of protection through which we can enforce that only the Batch front end and the Batch Driver can use the endpoints on the Batch Worker. This, along with #14581 are the final pieces to ensure that every endpoint in our system, both internal and external, uses HTTPS and performs the appropriate auth checks.

[daniel-goldstein]

This is mostly hooked up but not fully tested. I'd start by dev deploying this and submitting jobs, checking to see if any of the driver's requests to the worker (like in the schedule_job function) are rejected.