Remove dependency to `downward-api.machinepoollet.ironcore.dev` for filtering

[lukasfrank]

Summary

As in this discussion ( #1335 (comment) ) pointed out, relying on the downward-api.machinepoollet.ironcore.dev is not optimal.

The downward-api is only intended for ops purposes and can be configured to have a different name or it might be possible that the labels are not present at all (see: machine-downward-api-label).

Proposed change

At the moment we only store the parent Machine.UID in metadata.annotations.machinebroker.ironcore.dev/labels. We can additionally put the machinepoollet.ironcore.dev/machine-uid as a label to the Machine and utilize this for filtering.

Tasks

• Validate if this changes can work

Poollet:

• Update listMachinesByMachineKey to use again a machineUIDLabelSelector

Broker:

• Simplify ListMachines
  • Remove this check labelFilter := req.Filter; labelFilter != nil && labelFilter.LabelSelector != nil and handle it unified
• Update CreateMachine to set the label to the Machine