Add annotation for subnamespace when print tree
Add annotation for subnamespace when print tree

Tested: manually. The output looks like this:
  ```
  #setup
  $ k create ns root
  namespace/root created
  $ k create ns nonsub
  namespace/nonsub created
  $ k hns set nonsub --parent root
  Setting the parent of nonsub to root
  Succesfully updated 1 property of the hierarchical configuration of nonsub
  $ k hns create child -n root
  Successfully created "child" subnamespace anchor in "root" namespace

  #result
  $ k hns tree root
  root
  ├── [s] child
  └── nonsub
  [s] indicates subnamespaces.
  ```

kubernetes-sigs/multi-tenancy,kubernetes-sigs/multi-tenancy