Add labels to Shell node pod

[ruben-rodriguez]

Is your feature request related to a problem? Please describe.
Some environments often enforce labeling policies that doesn't allow to run workloads without certain mandatory labels. The current node shell pod is created without any label and it is not possible to customize them.

Describe the solution you'd like
I'd like to be able to configure the Shell node Pod metadata labels from the configuration like this

# $XDG_CONFIG_HOME/k9s/config.yml
k9s:
  clusters:
    # Configures node shell on cluster blee
    blee:
      featureGates:
        # You must enable the nodeShell feature gate to enable shelling into nodes
        nodeShell: true
      # You can also further tune the shell pod specification
      shellPod:
        image: cool_kid_admin:42
        namespace: blee
        limits:
          cpu: 100m
          memory: 100Mi
        labels:
          app.kubernetes.io/instance: k9s-node-shell
          app.kubernetes.io/managed-by: Ruben
          app.kubernetes.io/name: k9s-node-shell

Describe alternatives you've considered
#1847

Additional context
I'm sorry, I created the PR but forgot to submit this issue first 😅