Implement validating webhook for vm-operator VSphereMachine

[chrischdi]

/kind feature

Describe the solution you'd like

As follow-up from https://github.com/kubernetes-sigs/cluster-api-provider-vsphere/pull/2566#discussion_r1439493539 :

We should implement a validating webhook for vm-operator VSphereMachine to block in-place changes of the .spec fields (xref):

For VSphereMachine

• ImageName
• ClassName
• StorageClass
• MinHardwareVersion

Not to include ResourcePolicyName because that's a field on the VSphereCluster. Questionable if this should be get set immutable too 🤔

Anything else you would like to add:

Environment:

• Cluster-api-provider-vsphere version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[zhanggbj]

/assign