In some cases it can be advantageous to group several properties under a nested document in the serialized form.

For example, the class:

public class OrderItem
{
    public int OrderId { get; set; }
    public int ProductId { get; set; }
    public int Quantity { get; set; }
}

might need to be serialized as:

{
    _id :
    {
        OrderId : 1,
        ProductId : 2
    },
    Quantity : 3
}

While this serialization format could also be achieved by declaring a custom POCO to hold the OrderId/ProductId pair, there are cases where we might want the components of a compound primary key to appear at the top level in the POCO while being grouped together in a nested `_id` element in the serialized form.

We would like to support this option when using the `BsonClassMapSerializer`. This feature requires changes to the `BsonClassMapSerializer`, as well as adding code and attribute based ways to configure this.