Add Topic Fan-in

[isra17]

Create a topic that takes in parameter other topics to fan-in. This way a single jobdefinition could read from a single topic instead of having to define multiple topics and job definition. eg.

apiVersion: saturn.flared.io/v1alpha1
kind: SaturnTopic
metadata:
  name: fanin-topic
spec:
  type: FaninTopic
  options:
    topics:
      - type: PeriodicTopic
        name: periodic-topic
        options:
          interval: "* * * * * */5"
      - type: RabbitMQTopic
        name: rabbitmq-topic
        options:
          queue_name: foobar