Expose Node Count on HostedControlPlanes

User Story:

As a HyperShift controller in the management plane, I want to be able to:

• read the capacity of a cluster measured in the count of nodes from a HostedControlPlane or HostedCluster

so that I can achieve

• automation that reacts to the number of cluster workers changing at runtime

Acceptance Criteria:

Description of criteria:

• the count of nodes can be read from the HostedCluster or HostedControlPlane status

Engineering Details:

• design in https://docs.google.com/document/d/1VV1NZgrimSNFjJEH3PeZmUquey_YX3lm2l0FDwP8WDc/edit

This requires a design proposal.
This does not require a feature gate.