feat: add variables field to the v1beta3 eventtype spec

[Ahmed-Labs]

Fixes #7729

Proposed Changes

• Add EventVariableDefinitions to EventTypeSpec
• Validate whether variables used within attribute values are defined
• Update EventType v1beta3 CRD with added variables field

Pre-review Checklist

• 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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Ahmed-Labs / name: Ahmed Mohamed (6baa454, c6f6c9c, 7d00dad, 71b4b68)

[knative-prow]

Welcome @Ahmed-Labs! It looks like this is your first PR to knative/eventing 🎉

[knative-prow]

Hi @Ahmed-Labs. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[Cali0707]

Awesome start @Ahmed-Labs, thanks for working on this!

I left a few comments, and then I think this will be good to merge

[Cali0707]

/ok-to-test

[Cali0707]

/cc @pierDipi @matzew

FYI, this is the follow up regarding eventtypes with variables

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.16%. Comparing base (cdc6144) to head (71b4b68).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8445      +/-   ##
==========================================
+ Coverage   64.10%   64.16%   +0.06%     
==========================================
  Files         388      388              
  Lines       23363    23405      +42     
==========================================
+ Hits        14976    15018      +42     
  Misses       7591     7591              
  Partials      796      796              

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Ahmed-Labs
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[Cali0707]

/ok-to-test

[Cali0707]

@Ahmed-Labs can you fix the format errors?