Define ClusterTrustBundlePEM projected volume

This commit defines the ClusterTrustBundlePEM projected volume types.
These types have been renamed from the KEP (PEMTrustAnchors) in order to
leave open the possibility of a similar projection drawing from a
yet-to-exist namespaced-scoped TrustBundle object, which came up during
KEP discussion.

* Add the projection field to internal and v1 APIs.
* Add validation to ensure that usages of the project must specify a
  name and path.
* Add TODO covering admission control to forbid mirror pods from using
  the projection.

Part of KEP-3257.

Kubernetes-commit: ecfdc8fda55923c18708488ec1561a4fcf9f3e33