Extend the Pepr API to filter on additional AdmissionReview Objects

[cmwylie19]

Background::

The AdmissionReview object sent from kube-apiserver during admission contains a userInfo that contains username and groups.

Discussion::

Would it be fruitful to add Pepr filters based on those fields for admission objects?

When(UDSPackage).
isCreated()
.fromUsername("micah")
.inGroup("uds-admins")
.Validate(package=>package.Deny("Micah cannot create packages!"))

The thought is that this could potentially make Pepr stronger by requiring an action be performed by a specific user/belonging to specific group. It is similar to how RBAC works, but in this case the RBAC would delegate the decision to the module author.