WIP: Discovery: change behavior to allow ETs without any reference for better discovery

[matzew]

Fixes #7264

Feature track: https://docs.google.com/document/d/1US5Ve0CwhXwO8pUudN36vLkuSetEtSgwQ1btuG_9Xmk/edit

Proposed Changes

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

Release Note

Docs

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: matzew

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[knative-prow]

@matzew: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
unit-tests_eventing_main	98f5192	link	true	/test unit-tests

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[matzew]

/close
closing initial poc PR, due to PR #7423

[knative-prow]

@matzew: Closed this PR.

In response to this:

/close
closing initial poc PR, due to PR #7423

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