Allow versioning at path:method level

To facilitate the option of media-type versioning, it would be helpful to version at the path:method level. Here is the gist of my proposal: move version from root level to path:method level and represent as an array of versions. In making the change as described, my hope is that none of the existing functionality is sacrificed.

Single version example

/pets/{id}:
  put:
    tags:
    - "pets"
    versions:
    - version: "default"
      summary: "..."
      description: "..."
      ... all other properties formerly defined in the path-item

Multiple version example

/pets/{id}:
  put:
    tags:
    - "pets"
    versions:
    - version: "1"
      summary: "..."
      description: "..."
      operationId: "updatePet_v1"
      consumes:
      - "application/json"
      - "application/vnd.vendor.v1+json"
      deprecated: true
      ... all other properties currently defined in the path-item
    - version: "2"
      summary: "..."
      description: "..."
      operationId: "updatePet_v2"
      consumes:
      - "application/vnd.vendor.v2+json"
      ... all other properties currently defined in the path-item

The issue names no repository files, tests, or entry points; start by locating the path/method versioning model and its schema or validation tests. Done means supporting the proposed versions array at path:method level without removing existing functionality, including both single- and multiple-version examples.