Integrate with kube under a feature

[clux]

Initially as a hidden PoC to assess compatibility and see what unknown-unknowns we need to handle.

Expected work (all under a kube/pb feature):

• add k8s-pb as an optional dependency on a branch (pin a git dependency while working)
• implement kube::Resource for k8s-pb::Resource implementors in kube_core - can use HasMetadata as well (via Define and implement equivalent k8s-openapi traits #4)
• see if we can do querying outside protobufs
• try to add the envelope wrapper (via Handling Envelope Wrapper #6) so we can do actual protobuf queries in kube
• add a small example here to show how to use it (any example? #28)

If there are lacks on the k8s-pb side we can address it here as we go along.

Work we also should do (but could be avoided for a first PoC):

• figure out if we can feature select out k8s-openapi by using k8s-pb structs instead when in feature (lots of fiddly work)