[NR-250014] chore: unify reflectors

[sigilioso]

This PR removes duplicated code from k8s reflectors module:

Summary of changes

• DynamicObjectReflector and K8sControllerReflector<K> types are unified into Reflector<K>
• The namespace parameter is removed from the reflector builder (everywhere else we rely on the default namespace configured when building the client).

Out of scope

• Simplifying the k8s client: in order to make the review easier, dividing the k8s client responsibilities will be addressed in follow-up PRs.

[marcsanmi]

Thanks for addressing it 👍

[paologallinaharbur]

🚀