EdmDeltaEntityObject serialization issue

[vasrinwork]

This PR introduces a new EntitySet History:

**History** { string Id, string CustomerDelta }

where customerDelta is the serialized format of EdmEntityDeltaObject representation of Customer model.

On using a custom serialization technique, we see the below response for the GET History API:
Actual Response:

{
  "@context": "http://localhost:5048/odata/$metadata#History",
  "value": [
    {
      "Id": 1,
      "CustomerDelta": "{\"Id\":1,\"Name\":\"customer old name\",\"Orders\":[{\"Id\":1},{\"Id\":2},{\"Id\":1,\"Amount\":8,\"Items\":[{\"Id\":1}]}]}"
    }
  ]
}

Expected Response:

{
    "@context": "http://localhost:5048/odata/$metadata#History",
    "value": [
        {
            "Id": 1,
            "CustomerDelta": {
                "Id": 1,
                "Name": "customer old name",
                "Orders@delta": [
                    {
                        "Id": 1
                    },
                    {
                        "@removed": {
                            "reason": "deleted"
                        },
                        "@id": "http://tempuri.org/Orders(2)",
                        "Id": 2,
                        "Amount": 0
                    },
                    {
                        "Id": 1,
                        "Amount": 8,
                        "Items@delta": [
                            {
                            "Id": 1
                            }
                        ]
                    }
                ]
            }
        }
    ]
}

The serialization logic we used here omits the annotations/metadata like @removed, @context, @delta, etc.
We are trying to figure out if odata library provides any custom serialization methods that can convert the EdmDeltaEntityObject to a string, retaining the metadata and the annotations.