See the PR comment here: https://app.graphite.com/github/pr/10gen/mongo/53219/SERVER-125119-Refactor-%24graphLookup-to-populate-view-subpipelines-via-LiteParsed-layer#comment-PRRC_kwDOADUBOM7F_X_l

We should see if we can support this in IDL somehow, or rename the argument (with consideration to multiversion clusters) such that we don't have to manually operate on BSON, and we can rely on IDL - perhaps we can rely on the "cpp_variable" concept in IDL.