[Feature Request] Allow activity-only workers

[Sushisource]

Relates to https://github.com/temporalio/sdk-core/issues/224

A user has a need to limit the maximum number of activities targeting a certain resource and they do this by using many task queues. That's fine, but at the moment those workers would also poll (pointlessly) on workflow queues which introduces unnecessary burden. Allow running activity-only workers which do not poll on workflow queues at all.