Make SpecCluster declarative

`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.