Currently, the only way to make a DocumentSourceUnionWith with a specified pipeline is through DocumentSourceUnionWith::createWithBson which, of course, requires a BSON representation of the its sub-pipeline.

Previously, this has been fine, but for $rankFusion, we discovered a case where its problematic to create a DocumentSourceUnionWith from BSON, given that we already have a parsed Pipeine object.

We would like to add an additional DocumentSourceUnionWith constructor that can take in an already constructed Pipeline object. This will entail creating a new function with signature like:

DocumentSourceUnionWith(
  const boost::intrusive_ptr<ExpressionContext>& expCtx,
  NamespaceString unionNss,   
  std::unique_ptr<Pipeline, PipelineDeleter> pipeline);

The constructor implementation will look similar to the existing constructor that take the same argument, with the 'pipeline' as a std::vector<BSONObj>. Note you'll have to handle view resolution of the pipeline similar to how this constructor is (but not parse the pipeline). This will likely require copying over / similar logic to whats going on in buildPipelineFromViewDefinition / Pipeline::makePipelineFromViewDefinition.

Once this constructor is created, you should figure out how to unit test it (if possible). It would probably be smart to install the constructor into $rankFusion desugaring directly and $scoreFusion in this PR, as the existing hybrid search jstests tests passing both on a collection an view will prove out the correctness of the new constructor.