Define desired expressivity of Knative Eventing Trigger Filtering

[aslom]

Problem
Current trigger filtering expression are limited.

Persona:
Everybody?

Exit Criteria
Define level of expressivity and required performance/scaling levels.

Time Estimate (optional):
1w

Additional context (optional)
This is ongoing discussion and currently there is no one place that define what filtering should support and how.

[aslom]

I have created google doc where I tried to list existing efforts for improving filtering expressions with links to related issues and docs: https://docs.google.com/document/d/1bgM3xUQG722aZh_F6uGatYaLWXMU0s43xteQmcSYjHA/edit#

[grantr]

Related to #3359 and https://github.com/cloudevents/spec/blob/master/subscriptions-api.md

[github-actions]

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