refactor: PDB label selection

What this PR does / why we need it?

This PR changes the behavior of how kor maps unused PDBs. Currently, it validates the PDB label selector against Deployments/StatefulSets direct metadata labels, while it should validate the template labels, as PDBs select pods and through .metadata.ownerReferences field finds the respective workload controllers and not the other way around.

For example, the following Deployment is valid, but kor will map it as unused, as .metadata.labels field is not matched, even if the template labels match:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: app-name
spec:
  replicas: 2
  selector:
    matchLabels:
      app.kubernetes.io/name: app-name
      app.kubernetes.io/instance: app-instance
  template:
    metadata:
      labels:
        app.kubernetes.io/name: app-name
        app.kubernetes.io/instance: app-instance

In addition, added support for empty selectors & arbitrary pods.

If a PDB has an empty selector, it's now considered as used (matches every pod)

The behavior for an empty selector differs between the policy/v1beta1 and policy/v1 APIs for PodDisruptionBudgets. For policy/v1beta1 an empty selector matches zero pods, while for policy/v1 an empty selector matches every pod in the namespace.
If a PDB has an empty selector, but no running pods are found in the namespace, it'll be considered as unused.
PDBs support arbitrary pods (with no workload controllers), added validation for pods in addition to the Deployment/StatefulSet templates.

You can use a PDB with pods controlled by another resource, by an "operator", or bare pods

PR Checklist

[ ] This PR adds K8s exceptions (false positives)
[x] This PR adds new code
[x] This PR includes tests for new/existing code
[ ] This PR adds docs

GitHub Issue

Closes #364

Notes for your reviewers

$ kor pdb --show-reason
Unused resources in namespace: "empty"
+---+---------------+----------------+--------------------------------------------------------------+
| # | RESOURCE TYPE | RESOURCE NAME  |                            REASON                            |
+---+---------------+----------------+--------------------------------------------------------------+
| 1 | Pdb           | app-name-empty | Pdb matches every pod (empty selector) but 0 pods run        |
| 2 | Pdb           | app-name2      | Pdb is not referencing any deployments, statefulsets or pods |
+---+---------------+----------------+--------------------------------------------------------------+

Based the feature on the official PDB docs.

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Codecov Report

Attention: Patch coverage is 66.66667% with 26 lines in your changes missing coverage. Please review.

Project coverage is 43.38%. Comparing base (673c21e) to head (63f6aab). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/kor/pdbs.go	61.76%	17 Missing and 9 partials :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #366 +/- ## ========================================== - Coverage 44.48% 43.38% -1.11% ========================================== Files 63 63 Lines 3356 4020 +664 ========================================== + Hits 1493 1744 +251 - Misses 1622 2030 +408 - Partials 241 246 +5 ```

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yonahd / kor