Move eventing-contrib/cmd/* here

[vaikas]

The eventing-contrib/cmd tools were left around after the various other pieces moved to knative-sandbox/* repos. There have been no releases for those since Dec 2020. As agreed, we decided to move them into the knative/eventing repo so that we'll get proper updates / releases, etc.

Couple of notes:

• I did not update the copyright years, but instead kept them as is since they were moved from our own repo to here.
• I looked around for seeing if there was an easy way to release them into the eventing-contrib registry, but could not find one. This was discussed on slack and I agreed to take a look into it to see if it's possible, doesn't look like it. Plus I actually think it's cleaner if from the image you can easily see where it came from.

Proposed Changes

Pre-review Checklist

These are simple tools used for our docs examples that are simply moved here. I'll create the docs PR once I verify the release goes (I'll test with nightly) out correctly.

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature
• Conformance test for any change to the spec

Release Note

Tools from https://github.com/knative/eventing-contrib/tree/main/cmd were moved to eventing repo.

Docs
knative/docs#3489

[codecov]

Codecov Report

Merging #5295 (6c82a3d) into main (1f54d0e) will increase coverage by 0.30%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5295      +/-   ##
==========================================
+ Coverage   83.34%   83.64%   +0.30%     
==========================================
  Files         243      243              
  Lines        6879     6896      +17     
==========================================
+ Hits         5733     5768      +35     
+ Misses        797      785      -12     
+ Partials      349      343       -6     

Impacted Files	Coverage Δ
pkg/apis/messaging/v1/subscription_validation.go	80.00% <0.00%> (-1.40%)	⬇️
pkg/reconciler/broker/trigger/trigger.go	80.45% <0.00%> (+0.45%)	⬆️
pkg/reconciler/channel/channel.go	75.40% <0.00%> (+3.27%)	⬆️
pkg/broker/ingress/ingress_handler.go	75.75% <0.00%> (+12.99%)	⬆️
pkg/eventfilter/attributes/filter.go	100.00% <0.00%> (+13.63%)	⬆️
pkg/apis/messaging/v1/subscription_defaults.go	100.00% <0.00%> (+66.66%)	⬆️

---

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[vaikas]

Lint fails with:

  Error: SA1019: cloudevents.NewDefaultClient is deprecated: Please use NewClientHTTP with the observability options.  (staticcheck)
  Error: SA1019: cloudevents.NewDefaultClient is deprecated: Please use NewClientHTTP with the observability options.  (staticcheck)
  Error: SA1019: cloudevents.NewDefaultClient is deprecated: Please use NewClientHTTP with the observability options.  (staticcheck)

[matzew]

tested the event-display with the png source:

k logs -f -l serving.knative.dev/service=event-display -c user-container                        
  specversion: 1.0
  type: dev.knative.sources.ping
  source: /apis/v1/namespaces/default/pingsources/ping-source
  id: 93439c6d-0f05-45b3-85c2-78ca77cd64a6
  time: 2021-04-22T06:57:00.20280561Z
  datacontenttype: application/json
Data,
  {
    "message": "Hello world!"
  }

/lgtm
/approve

[knative-prow-robot]

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