Currently, the ParseNode interface expansion requires making two calls across the API to expand.

In subsequent iterations of our designs, we landed on ArrayContainer design, which separates the concerns of array size and transfer of the array elements across the boundary into a dedicated abstraction.

ParseNode may benefit from this abstraction as well. 

As part of this ticket, re-implement expansion using this approach: https://github.com/10gen/mongo/blob/ccd36dcaebc31452905f729cd9110490c97a2be1/src/mongo/db/extension/sdk/dpl_array_container.h#L77