Support all work pool types

[chrisguidry]

In the current state, work pools may be of type process or kubernetes (due to a quick shortcut/hack to use the -kubernetes prefect image). Any other type of work pool would require a custom image or the PIP_EXTRA_PACKAGES setting to load the additional integration packages with the correct worker implementations.

We should be able to support any kind of Prefect-maintained work pool declaratively by arranging for the installation of the appropriate integration package. For example, when using ecs, we'd arrange for prefect-aws to be installed in the worker, etc. The general-purpose way to accomplish this would be by reading the global collection catalog and installing (via PIP_EXTRA_PACKAGES?) the correct Prefect integration.