Allow extensions to modify resource models

[mantis-toboggan-md]

Is your feature request related to a problem? Please describe.
In order to support the virtual clusters ui extension, we need to be able to modify resource models through extensions. For this specific extension we would like to be able to add the following to the provisioning cluster model:

• modify a resource's available actions (potentially removing some, not just adding as we currently can)
• define before/after save/delete hooks

Describe the solution you'd like
#11064 includes a proposed solution - we should make sure this is the direction we want to take for modifying resource models via extension more generally