Be able to easily define branches

[lionelvillard]

Problem
It would be nice to easily define mutually exclusive branches.

     |->C1->F1
     |->C2->F2
     ...

The F1 branch is invoked only when the condition C1 is true. The F2 branch is invoked only when C1 is false and C2 is true.

This is similar to sequence.

Persona:
Which persona is this feature for?
Event Consumer

Exit Criteria
A measurable (binary) test that would indicate that the problem has been resolved.
When a user can easily create branches and plug it into the broker.

Time Estimate (optional):
How many developer-days do you think this may take to resolve?
a day

Additional Context:
https://docs.google.com/document/d/17_6ONVtE397X_H5J0mlzvJKiqFWzoHLOqapis6f7nlo/edit#

[lionelvillard]

The current proposed implementation #1529 defines a new CRD Choice with cases and filter, eg:

apiVersion: messaging.knative.dev/v1alpha1
kind: Choice
metadata:
  name: check-assignment
spec:
  channelTemplate:
    apiVersion: messaging.knative.dev/v1alpha1
    kind: InMemoryChannel
  cases:  
  - filter:
      ref:
         apiVersion: serving.knative.dev/v1alpha1
         kind: Service
         name: data-assigned
    subscriber:
      ref:
        apiVersion: serving.knative.dev/v1alpha1
        kind: Service
        name: send-notice

Here some naming alternatives:

1. Routes, choices, predicate
2. Routes, choices, when
3. Branches, routes and filter
4. others?

I like 1.