Add support for the impersonate RBAC verb

[raesene]

What would you like to be added

At the moment the impersonate RBAC verb does not appear to be supported.

kubectl-who-can impersonate serviceaccounts
Error: resolving resource: the "serviceaccounts" resource does not support the "impersonate" verb, only [create delete deletecollection get list patch update watch]

Adding this is a bit trickier as, according to the documentation impersonate can be set on resources which don't actually exist in etcd (specifically users and groups)

Why is this needed

Adding impersonate would be useful as it can allow for privilege escalation (similarly to escalate, and bind).