Event Discovery: Add Variables to EventType V1Beta3 spec

[Cali0707]

Problem
In the new v1beta3 api for eventtypes, we are allowing users to place "variables" into template strings for the various eventtype attributes. It would be good to allow users to give more information about what values are allowed for those variables.

To accomplish this, we can add a variables section to the spec like this:

apiVersion: eventing.knative.dev/v1beta3
kind: EventType
spec:
  reference:
    ...
  attributes:
    - name: "type"
      value: "event.{requestStatus}.foo"
      required: true
    ...
  variables:
    - name: "requestStatus"
      regex: "[a-zA-Z]"
      example: "request.failed"

[Cali0707]

cc @pierDipi @matzew @dsimansk

I'd love to get some feedback on this idea

[Cali0707]

@pierDipi can we discuss this at the WG call this week? I need a decision on how we are planning to proceed here for #7883