Flatten `path_bindings` into a list of strings (aux graph IDs)

[colleenXu]

[EDIT: now being considered for TRAPI 2.0, which doesn't need backwards-compatibility]

In 1.6-beta, the path_bindings values (defined using additionalProperties) are arrays of PathBinding objects. The PathBinding object only has 1 defined property, id.

If we aren't taking advantage of the PathBinding object structure (having other properties besides id), it would be more concise and simple to make the path_bindings value an array of strings (aux graph ids) instead.

Previous ideas for how to change the schema to a list of aux graph ids:

• Option A for flattening path_bindings #502: removes PathBinding class
• Option B for flattening path_bindings #503: adjusts PathBinding class

---

Current 1.6-beta spec:

            path_bindings:
              type: object
              description: >-
                The dictionary of input Query Graph paths to Analysis paths, specifically only for pathfinder queries.
              additionalProperties:
                type: array
                items:
                  $ref: '#/components/schemas/PathBinding'
...
    PathBinding:
      type: object
      description: >-
        A instance of PathBinding is a single binding of an input QueryGraph path
        (the key to this object) with the AuxiliaryGraph id containing a list of
        edges in the path. The Auxiliary Graph does not convey any order of edges
        in the path.
      properties:
        id:
          type: string
          description: The key identifier of a specific auxiliary graph.
      additionalProperties: true
      required:
        - id