[release-0.26] 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]

Codecov Report

Merging #5778 (49a1565) into release-0.26 (f031ba2) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff                @@
##           release-0.26    #5778      +/-   ##
================================================
+ Coverage         82.65%   82.67%   +0.01%     
================================================
  Files               202      202              
  Lines              6326     6332       +6     
================================================
+ Hits               5229     5235       +6     
  Misses              758      758              
  Partials            339      339              

Impacted Files	Coverage Δ
pkg/apis/messaging/v1/channel_validation.go	84.21% <100.00%> (+1.35%)	⬆️
pkg/apis/messaging/v1/subscription_validation.go	86.04% <100.00%> (+1.04%)	⬆️

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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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[pierDipi]

/retest

[travis-minke-sap]

@googlebot I consent