Adding support for named integer enums

[barrybarrette]

Adding support for named enums via an optional extension, x-enum-varnames.

This extension is added to the schema inline with the enum definition:

 "Siva.e_Install_State": {
        "enum": [
          0,
          1,
          2,
          3,
          4,
          5,
          6,
          99
        ],
        "type": "integer",
        "format": "int32",
        "x-enum-varnames": [
          "Deinstalled",
          "Installed",
          "Upcoming_Site",
          "Lab_Site",
          "Pending_Deinstall",
          "Suspended",
          "Install_In_Progress",
          "Unknown"
        ]
      },

The result:

[dbanty]

I've resisted adding custom OpenAPI attributes just for this generator... ideally, we could adopt conventions used by more popular projects, like OpenAPITools. Is x-enum-varnames used elsewhere already?

[barrybarrette]

ideally, we could adopt conventions used by more popular projects, like OpenAPITools. Is x-enum-varnames used elsewhere already?

Yes, OpenAPITools adopted it in this PR -> OpenAPITools/openapi-generator#917

I did also search around for any other official or even adopted standard for this but came up empty.

[barrybarrette]

Just fixed the possible null reference that failed tests. Also added a sanity check to ensure the lengths of the var_names matches the length of the actual values, inspired by the OpenAPITools implementation.