Add querying functionality for valid worker versions that may be specified for a job request

[robertbartel]

Related to #662, add further capabilities to let users list the available valid options for worker versions that can be specified. Given the current design dependent on Docker, this basically would be querying the deployment-configured Docker registry (either directly or via the request service) for what image tags can be used. However, we should also consider things more generally than just the current design ...

This likely has practical implications on and/or constraints from:

• plans to modify the general configuration setup for DMOD (things like Refine docker network configuration #562 and Refactor/Simplify Docker swarm/compose configurations #651)
• plans to add support for infrastructure backends other than Docker Swarm (see Abstract Docker away from DMOD and make it an optional dependency #703 and subtasks).