Support `asyncapi` `mqtt` proxy using `asyncapi.yaml`

[jfallows]

Describe the desired outcome from the user's perspective
As an AsyncAPI consumer, I want to configure zilla as mqtt proxy using asyncapi.yaml more directly in zilla.yaml.

Acceptance criteria

• asyncapi server binding configured via asyncapi.yaml for mqtt
• asyncapi client binding configured via asyncapi.yaml for mqtt
• only asyncapi server binding and asyncapi client binding needed to configure asyncapi mqtt proxy

Additional context
Syntax similar to the following in zilla.yaml using composite bindings.

bindings:
  asyncapi_server:
    type: asyncapi
    kind: server
    options:
      specs:
        - mqtt/asyncapi.yaml
    exit: asyncapi_client0
  asyncapi_client:
    type: asyncapi
    kind: client
    options:
      specs:
        - mqtt/asyncapi.yaml