[release-1.19] Fix mt-broker-ingress auth to work with structured event too

[knative-prow-robot]

This is an automated cherry-pick of #8710

[codecov]

Codecov Report

❌ Patch coverage is 43.24324% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.46%. Comparing base (931f60b) to head (e8aa563).
⚠️ Report is 1 commits behind head on release-1.19.

Files with missing lines	Patch %	Lines
pkg/utils/utils.go	57.14%	6 Missing and 3 partials ⚠️
pkg/channel/event_receiver.go	25.00%	5 Missing and 1 partial ⚠️
pkg/broker/ingress/ingress_handler.go	28.57%	4 Missing and 1 partial ⚠️
pkg/auth/verifier.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.19    #8713      +/-   ##
================================================
+ Coverage         51.42%   51.46%   +0.04%     
================================================
  Files               400      400              
  Lines             25145    25158      +13     
================================================
+ Hits              12930    12948      +18     
+ Misses            11433    11424       -9     
- Partials            782      786       +4     

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[Cali0707]

/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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