Implement alpha tree command for Machines and Volumes

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Summary

Implement a command showing tree output for MachinePools / StoragePools.

The command should accept an optional MachinePool / StoragePool name and show all Machines / Volumes assigned to that pool.

Caution: In the future, this might be extended to additionally include volumes that are synced to that pool via its items. An example for this could be a machine also including the Volumes it references.

Basic example

$ kubectl-onmetal alpha tree machinepool

- machine-pool-a
  - machine-a
  - machine-b
- machine-pool-b
  - machine-c
  - machine-d
- <unassigned>
  - machine-e
  - machine-f

$ kubectl-onmetal alpha tree machinepool machine-pool-a
- machine-a
- machine-b

Definition of Done

• The command described as above is implemented
• There are tests for all new code paths
• The PRs pass / the author should fix all workflows / his tests to make sure they pass