Add defaulting webhook for shoot resource

[Gerrit91]

I think we should only default fields of a provider configuration in case the entire configuration is nil. Otherwise just keep it as it is. This is safer for backwards-compatibility and also allows leaving some fields empty on purpose.

[Gerrit91]

Needs rebase

[mwindower]

For cilium we need additional properties in networking.providerConfig like this:

networking:
    nodes: 10.128.16.0/22
    pods: 10.240.0.0/13
    providerConfig:
      apiVersion: cilium.networking.extensions.gardener.cloud/v1alpha1
      devices:
      - lan+                                                                                             
      hubble:                     
        enabled: true
      ipv4NativeRoutingCIDREnabled: true 
      kind: NetworkConfig
      loadBalancingMode: dsr                                                                             
      mtu: 1440                           
      psp: true       
      tunnel: disabled
    services: 10.248.0.0/18
    type: cilium