Add (better) support for Crossplane clusters

[bcle]

This may involve adding a new ApiProvider property to the clusterspec object.

Aha! Link: https://pf9.aha.io/features/ARLON-170

[bcle]

There is current experimental support for Crossplane clusters, but they suffer from 2 major issues I've observed but haven't been able to investigate fully:

• ArgoCD flags some created resources in the tree as out of sync or unhealthy. I think it's due to Crossplane compositions having 2 flavors of many objects: the actual resources, and "claims" on the resources. This seems to confuse ArgoCD.
• 90% of the resources (and the underlying infrastructure elements) tend to get automatically deleted after half a day. I have no idea why this is happening. It could be related to the first bullet, combined with some garbage collection.

[cruizen]

@cre8minus1 , @bcle I have added this to the icebox for now assuming we are not taking it up any time soon.