[virtual-kubelet] reflect pod RuntimeClass

[fra98]

Description

This PR introduces the support for pod RuntimeClass reflection.

The RuntimeClass (.spec.runtimeClassName field) is reflected from the local pod to the remote one.

If you are using the Liqo RuntimeClass, you cannot specify the RuntimeClass name as the field is already used. To overcome this problem, you can annotate the pod with liqo.io/remote-runtime-class-name: <MY_RUNTIMECLASS_NAME>.

It is also possible to enforce a remote RuntimeClass to all pods scheduled on a virtual node, by specifying it in the OffladingPatch of the virtualnode (.spec.offloadingPatch.runtimeClassName).
If you are using liqoctl to create the virtual node, you can leverage the --runtime-class-name flag.

If these options are used in combination, the following priority (from higher to lower priority) will be used to determine the remote RuntimeClass:

1. pod Annotation (liqo.io/remote-runtime-class-name).
2. pod RuntimeClass (.spec.runtimeClassName). It is ignored if set to liqo.
3. virtualNode OffloadingPatch (.spec.offloadingPatch.runtimeClassName).

[adamjensenbot]

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[fra98]

Fix #2869