Support `openapi` `http` proxy using `openapi.yaml`

[jfallows]

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

Acceptance criteria

• openapi server binding configured via openapi.yaml
• openapi client binding configured via openapi.yaml
• only openapi server binding and openapi client binding needed to configure openapi proxy

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

bindings:
  openapi_server:
    type: openapi
    kind: server
    options:
      specs:
        - http/openapi.yaml
    exit: openapi_client0
  openapi_client:
    type: openapi
    kind: client
    options:
      specs:
        - http/openapi.yaml