Add support for specifying userData and networkData as part of the worker config

[stoyanr]

Currently, Kubevirt VM userData is configured using a sophisticated mechanism in which multiple extensions participate. networkData is hardcoded in https://github.com/gardener/machine-controller-manager-provider-kubevirt to the following YAML snippet if custom networks are used, and to an empty string otherwise:

version: 2
ethernets:
  id0:
    match:
      name: "e*"
    dhcp4: true

We should be able to specify user-defined parts of the userData and the networkData in the Shoot resource as part of the worker config, as it could be used to configure other aspects beyond what is already configured that could be important in certain cases. See Cloud config examples and Networking Config Version 2 for the full documentation.

The contents of userData and networkData specified by the user should be merged with the already computed / default configurations. TODO How should conflicts be resolved?

Example Shoot resource with userData and networkData configured:

spec:
  provider:
    type: kubevirt
    workers:
    - name: worker1
      ...
      providerConfig:
        apiVersion: kubevirt.provider.extensions.gardener.cloud/v1alpha1
        kind: WorkerConfig
        userData: |
          <...>
        networkData: |
          <...>

/area networking
/kind enhancement
/priority normal
/platform kubevirt