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

[jfallows]

We often test via spec scripts to verify implementation behavior instead of tightly coupled unit tests.
The intent is to be able to refactor the code, but still have stable tests to verify new implementation. Tightly coupled tests are typically forced to change when the code is refactored, preventing the stable basis to verify consistency.

Let's discuss further over Slack community as needed on this feedback.

[jfallows]

See HttpOptionsConfigAdapter for an example of a fairly complex object adapter so you can see the coding patterns.