Make SpecCluster declarative

[jacobtomlinson]

SpecCluster and other components within distributed like distributed.cli.dask_spec aim to create clusters and workers according to a specification.

Currently these operations are performed imperatively. The SpecCluster class is given a spec and a resource is created to match the spec.

I would like to move things towards a declarative approach instead. This would mean the class is given a spec and it ensures that a resource exists to match that spec. If one doesn't exist it will create it, same as today. But if it already exists it will do nothing.

This slight difference of ensuring a resource instead of creating one will enable much more flexibility when it comes to recreating objects to represent existing resources. This ties into the lifecycle work I've been doing as part of #4263.

Opening this issue to signal my intent to work on this.