operator: Extract cilium-pod selector into a dedicated cell.Config

`CiliumK8sNamespace` and `CiliumPodLabels` are consumed by three cells:
node taint sync, kvstore nodesgc, and ingress, with each site duplicating
parts of the namespace-default resolution and label-selector parsing.

This PR introduces a small `cilium-pod-config` cell holding the two fields
plus helper methods `ResolveNamespace(fallback)` and `Selector()` and
feed those to the 3 cells using them via hive dependency injection.

Signed-off-by: Hadrien Patte <hadrien.patte@datadoghq.com>

Author: HadrienPatte