add a script to print summary of images used by jobs

[spiffxp]

it should probably turn into something not-bash if I'm going to lean on it too hard, but I found it useful for scoping out kubernetes/k8s.io#1523 and want to keep it somewhere shared

[spiffxp]

/cc @chaodaiG @cjwagner
this is how I generated the snippet I shared
/cc @BenTheElder
E_TOOMUCHBASH

Sample output

# images used by prowjobs on prow.k8s.io
- total /                      2328 total, 53 unique
  - not gcr.io /               111 total, 13 unique
    - dockerhub /              106 total, 9 unique
    - quay.io /                5 total, 4 unique
  - gcr.io /                   2217 total, 40 unique
    - kubernetes.io
      - k8s-staging            22 total, 7 unique
      - k8s.gcr.io             10 total, 3 unique
    - google.com
      - k8s-prow               13 total, 6 unique
      - k8s-testimages         2148 total, 14 unique
        - kubekins-e2e         1867 total, 1 unique
        - image-builder        138 total, 1 unique
        - krte                 61 total, 1 unique
        - other                220 total, 12 unique
          - bazelbuild         5 total, 1 unique
          - benchmarkjunit     4 total, 1 unique
          - bigquery           2 total, 1 unique
          - bootstrap          9 total, 1 unique
          - e2e-kubemci        1 total, 1 unique
          - gcloud-bazel       23 total, 1 unique
          - gcloud-in-go       13 total, 1 unique
          - generic_autobump   2 total, 1 unique
          - gubernator         1 total, 1 unique
          - image-builder      138 total, 1 unique
          - krte               61 total, 1 unique
          - bazel              5 total, 1 unique
          - triage             1 total, 1 unique
    - other                    24 total, 10 unique
      - bazel                  25 total, 3 unique
      - ci                     2 total, 1 unique
      - mdlint                 3 total, 1 unique
      - shellcheck             3 total, 1 unique
      - minikube-e2e           1 total, 1 unique
      - minigo-prow-harness-v2 4 total, 1 unique
      - gcloud-bazel           24 total, 2 unique
      - bazel                  25 total, 3 unique
      - cip                    5 total, 1 unique
      - octodns                1 total, 1 unique

[chaodaiG]

Since it serves its purpose, not going to nitpicking on bash styling. I think you should update the copyright header

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