Provide an overload of PostDocumentAsync that allows specifying a different type for new document

[DiscoPYF]

I have a use case where I would like to be able to provide a different type for the newly returned document.

Given existing classes MyModel and MyModelNew containing only a subset of properties from MyModel. I would like to be able to call PostDocumentAsync to post a MyModelNew but return a MyModel. We support this with PatchDocumentAsync.

Using ignore null values values isn't an option because there are null values I would like to post. I could use serialization attributes but provide a new method supporting this use case would be much simpler.

I propose to add PostDocumentAsync<T, U> where T is the type to post and U the type to return in PostDocumentResponse.New, when PostDocumentsQuery.ReturnNew is specified.

Example of use:

var response = await client.PostDocumentAsync<MyModelNew, MyModel>(
    "MyClxName",
    instanceOfMyModelNew,
    new PostDocumentsQuery()
    {
        ReturnNew = true
    });

MyModel fullModel = response.New;