Structural improvements: enhance consumes and produces handling

[arno-di-loreto]

Following my ideas from OAI/sig-moonwalk#115 and extending the #660 idea, we could modify consumes and produces handling.

On spec's current version, you handle the produced and consumed media types this way:

Produced and consumed media type can be declared on root level for all paths and operations:

consumes:
  - application/json
produces:
  - application/json

And they can be overridden on operation level:

paths:
  /resources:
    get:
      consumes:
        - application/xml
      produces:
        - application/xml

The idea is to enable definition/overriding at each level (whole API, path, operation) for both produces and consumes and also add produces on response level (as proposed in #660)

paths:
  consumes:
    # apply to all path and operations
  produces:
    # apply to all path and operations
  /resources:
   produces:
     # apply to this path's operations and override upper level
   consumes:
     # apply to this path's operations and override upper level
   get:
     produces:
       # apply to this operation and override upper levels
     consumes:
       # apply to this operation and override upper levels
     responses:
       200:
         produces:
           # apply to this response and override upper levels

[arno-di-loreto]

parent #560

[MikeRalphson]

produces/consumes were removed/reworked in v3.0 and I think the content approach covers the issue here.