Validating webhook handler should collect all errors, instead of returning the first one found

[dlipovetsky]

We're using controller-runtime's MultiValidatingHandler, which returns as soon as one validator reports an error*. That prevents other validators from running, and the user may get an incomplete picture of validation failures.

cluster-api-runtime-extensions-nutanix/pkg/webhook/cluster/validator.go

Lines 11 to 16 in ffb7bb2

	func NewValidator(client ctrlclient.Client, decoder admission.Decoder) admission.Handler {
	return admission.MultiValidatingHandler(
	NewClusterUUIDLabeler(client, decoder).Validator(),
	NewNutanixValidator(client, decoder).Validator(),
	)
	}

• I raised this behavior in MultiValidatingHandler returns as soon as one handler fails, making it impossible to follow convention of reporting all validation errors kubernetes-sigs/controller-runtime#3203

[dlipovetsky]

I'd like to fix this by replacing our use of MultiValidatingHandler.