When an extension Desugar stage expands into a stage that is not implemented by the extension, the extension must be able to provide a node in the ExpansionArray which the Host can later parse into the actual runtime implementation, either a LiteParsedDocumentSource, or a DocumentSource depending on where expansion is taking place. 

For this purpose, the Extensions API will provide a function (create_host_stage_parse_node) which will permit extensions to create a ParseNode representing an extension stage which the extension cannot implement (i.e $match). 

// Addition to MongoExtensionHostPortal
typedef struct MongoExtensionHostPortalVTable {
/**
 * create_host_stage_parse_node: Creates a Host implemented ParseNode for a stage which 
 * the extension does not implement. 
 **/
MongoExtensionStatus* (*create_host_stage_parse_node)(MongoExtensionByteView bsonSpec,
                                          MongoExtensionAggregationStageParseNode** node);
} MongoExtensionHostPortalVTable;

Add this function to the public API, and add unit tests for it.