[release-0.24] Validate DeliverySpec in Channels & Subscriptions

[knative-prow-robot]

This is an automated cherry-pick of #5777

Channel and Subscription will now enforce validation of the "delivery" field.

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Codecov Report

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@@               Coverage Diff               @@
##             release-0.24    #5780   +/-   ##
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  Coverage                ?   82.66%           
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  Files                   ?      198           
  Lines                   ?     6168           
  Branches                ?        0           
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  Hits                    ?     5099           
  Misses                  ?      742           
  Partials                ?      327           

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[pierDipi]

/lgtm
/approve
/assign @travis-minke-sap

for CLA consent

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, pierDipi

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[travis-minke-sap]

@googlebot I consent