MQTT topics with path parameters required to match guarded identity

[epieffe]

The goal of this PR is to allow secure access to some MQTT client specific topics for publish and/or subscribe.

For publish or subscribe routes:

• mqtt server can define path parameters for any segment in a topic name
• mqtt server can enforce that a path parameter matches the client's guarded identity (e.g. via jwt guard)

Example configuration:

mqtt_server0:
  type: mqtt
  kind: server
  routes:
    - when:
        - publish:
          - topic: taxi/{id}/location
            params:
              id: ${guarded['jwt'].identity}
        - subscribe:
          - topic: taxi/{id}/update
            params:
              id: ${guarded['jwt'].identity}
      exit: mqtt_kafka_proxy0

Fixes #1382