Cluster should support many worker types

The various Cluster objects often allow the user to provide specifications of a worker (cores, memory, software environment, ...) and then provides mechanisms around increasing and decreasing the number of workers.

However sometimes a dask deployment has a few different kinds of workers, for example machines with GPUs or high memory, or machines from a queue that is more or less expensive or reliable in some way.

This suggests that maybe the Cluster object should accept a list of worker pools, and provide common functionality around them.

Things like the widget are easy to scale to multiple pools. Adaptivity is a bit weirder.

cc @lesteve @jhamman (dask-jobqueue) @jcrist (dask-yarn) @jacobtomlinson (dask-kubernetes)

Credit for this thought goes to @lesteve