Support finding ingresses

[Yuni-sa]

to solve #27
check for unused ingresses by checking if services exist, can check also for services in defaultBackend, if ingress points to a resource mark it as used.

[Yuni-sa]

after some more testing I caught a bug where resources are skipped like with this manifest:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: test-ingress
spec:
  rules:
    - host: example.com
      http:
        paths:
          - path: /app1
            pathType: Prefix
            backend:
              service:
                name: service-that-doesnt-exist
                port:
                  number: 8080
          - path: /resource
            pathType: ImplementationSpecific
            backend:
              resource:
                apiGroup: k8s.example.com
                kind: StorageBucket
                name: assets

[yonahd]

LGTM.
Thanks for the contribution!