Support xs-ms-enum values without the "value" label  #216

Some contract generators define the values of x-ms-enum as a list of actual values without labels.
The current version OpenApi-Diff doesn't support this format and throws a Newtonsoft.Json.JsonSerializationException in this case.

Example of x-ms-enum with and without the "value" label
"enum": [ "Mammals", "Fish", "Birds", "Reptiles", "Amphibians", "Invertebrates" ], "type": "string", "x-ms-enum": { "name": "PetType", "modelAsString": false, "values": [ { "value": 0, "description": "humans and all other animals that are warm-blooded vertebrates", "name": "Mammals" }, { "value": 1, "description": "aquatic, craniate, gill-bearing animals that lack limbs with digits" }, { "value": 2, "name": "Birds" }, 3, 4, 5 ], "x-nullable": false }

The issue with this XmsEnumExtension is that we cannot cast an object of primitive type (e.g. long, string) to a XmsEnumValue , thus all contracts that contains x-ms-enum with a list of values of primitive type causes a deserialization issue in the OpenApiDiff comparison.

The exception is the following

Newtonsoft.Json.JsonSerializationException : Error converting value 3 to type 'AutoRest.Swagger.Model.XmsEnumValue'. Path 'definitions.PetType.x-ms-enum.values[0]', line 235, position 11.
  ----> System.ArgumentException : Could not cast or convert from System.Int64 to AutoRest.Swagger.Model.XmsEnumValue.