eventing: Expression language for Trigger filter

Problem Now the Trigger filtering abilities are poor, because they require direct match on specific CloudEvent attributes.

Persona: User

Exit Criteria Trigger APIs should have a more powerful filtering abilities

Time Estimate (optional): 2

Additional context (optional) Together with @grantr we discussed the possibility to add an expression language to express the matching predicate. We need to find an expression language with implementations in different programming languages and that covers the basic string operations.

An example trigger could be:

apiVersion: eventing.knative.dev/v1beta1
kind: Trigger
metadata:
  name: my-service-trigger
spec:
  broker: default
  filter:
    predicate: my-extension-value == dev.knative.foo.bar || type == dev.knative.foo.bar
  subscriber:
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: my-service