[release-1.18] 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 41.66667% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.34%. Comparing base (d952cdf) to head (1059fc2).

Files with missing lines	Patch %	Lines
pkg/utils/utils.go	57.14%	6 Missing and 3 partials ⚠️
pkg/channel/event_receiver.go	14.28%	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-1.18    #8714      +/-   ##
================================================
+ Coverage         52.31%   52.34%   +0.03%     
================================================
  Files               397      397              
  Lines             24925    24937      +12     
================================================
+ Hits              13039    13053      +14     
+ Misses            11099    11092       -7     
- Partials            787      792       +5     

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[Cali0707]

/lgtm

[knative-prow]

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