enhancements/security/openshift-image-policy: Propose new enhancement

Increasing the security of OpenShift release images from the current
"check GPG signatures before initiating an update" to "check Sigstore
signatures on every Pod launch".

Like the (Cluster)ImagePolicy enhancement I'm building on, this
enhancement mostly belongs to the Node component.  But the node
component doesn't seem to have its own subdirectory, so I'm dropping
this into the enhancements/security directory for now.