Add API to allow operator to add custom system workflows to worker role

Operators of Temporal often have their own system-level workflows they run in order to support various integrations with the underlying infrastructure or org-specific enterprise systems. We can always spin up our own worker apps, but in some cases it would be simpler to just piggyback on top of the existing temporal-server worker role.

The Temporal server code already supports many ways to customize the server code via ServerOptions (or dependency injection in the latest versions?). I'm thinking it would be nice to allow for a new customization API that would allow the operator to specify system level workflows and corresponding workers that should be run within the worker role.

We would probably need the ability to specify the workflow id of these custom workflows. We would also need to be able to specify some of the same sets of options as are currently specified in the Config struct in the worker package.