Make server act as "default webhook"

[found-it]

Right now the server just adds the edera runtime class. We shouldn't obliterate the existing runtime class if there is one and instead only operate on pods that don't have a runtime class

[found-it]

See the developing guide, when it merges (#32) for how to spin up the server locally. The server will take an admission request in json format and will return a json patch, like so

➜ curl -sk -H'content-type: application/json' -d @data/admission.json https://0.0.0.0:8443/mutate | jq -r .response.patch | base64 -d | jq
[
  {
    "op": "add",
    "path": "/spec/runtimeClassName",
    "value": "edera"
  }
]

In this issue we should figure out what the admission.json looks like when there is a runtimeClass set already (likely just backing out where it's set in the pod spec). Then adding the appropriate structs to extract the runtimeClass from admission.json and then having the server check if there is something set or not before issuing a json patch.

Ideally the "default-ness" is configurable for users so they can enable/disable the behavior