Add field in ClusterVersion spec to request the target architecture

Add spec field in ClusterVersion spec to request the target architecture i.e. heterogeneous or homogeneous arch.

Design choices:

• What is the new property name, and where does it live in the spec hierarchy?  spec.desiredCPUArchitecture?  spec.desiredUpdate.cpuArchitecture?  Other?

• What are the new property values?  The GOARCH strings (amd64, etc.) or ART strings (x86_64, etc.)?  Multi or other for the heterogeneous case?

• When an admin sets this property, that is a request to make the transition as soon as the CVO can.  That is distinct from how channel works today, where channel=stable-4.12 in a 4.11 cluster is "I am interested in hearing about updates to 4.12, and once I see recommendations, I may or may not decide to accept those recommendations".  This new property will be a single "I am interested in making the transition as soon as possible, don't ask me again".

 * What happens when the new property and desiredUpdate.image are both set, but have incompatible values?  E.g. the new property is Multi but the image pullspec is amd64? Which does the CVO prefer? Does the CVO complain about the incompatibility in status? If so, how (particular conditions[].type? Other?

Definition of done:

• Create tickets to implement the selected design decisions.

Enhancement: https://github.com/openshift/enhancements/blob/master/enhancements/multi-arch/heterogeneous-architecture-clusters.md