Add `fidesops worker` command to run Celery workers

[seanpreston]

This works in development, but for a production deployment we need to make the celery worker configuration more foolproof

Can we add a new CLI command like fidesops worker, which acts similarly to fidesops webserver except it starts celery instead of uvicorn?

Originally posted by @NevilleS in #621 (comment)

[adriaaaa]

@conceptualshark do we document CLI commands like this in our docs? If so, we may need to add this ticket to your doc board