Support `mqtt` to `kafka` 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 to kafka using asyncapi.yaml more directly in zilla.yaml.

Acceptance criteria

• asyncapi server binding configured via mqtt asyncapi.yaml
• asyncapi client binding configured via kafka asyncapi.yaml
• asyncapi proxy binding configured via mqtt and kafka asyncapi.yaml
• only asyncapi server, proxy and client bindings needed to configure mqtt to kafka 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_proxy
  asyncapi_proxy:
    type: asyncapi
    kind: proxy
    options:
      specs:
        - mqtt/asyncapi.yaml
        - kafka/asyncapi.yaml
    routes:
      - when:
           - operation: ... # mqtt asyncapi operation id
         exit: asyncapi_client
         with:
           operation: ... # kafka asyncapi operation id
  asyncapi_client:
    type: asyncapi
    kind: client
    options:
      specs:
        - kafka/asyncapi.yaml