In order to restrict the usage of $_internalSearchIdLookup by extensions, we need to provide a way for extensions to allocate an AstNode for Idlookup which extensions can expand into.

In order to do so, we need to implement a host::AstNode which wraps a LiteParsedDocumentSource for $_internalSearchIdLookup. Currently, no such stage exists, instead, we use LiteParsedSearchStage, which is not quite correct. 

As part of this ticket, implement a LiteParsedDocumentSourceInternalSearchIdLookup which can be used in the host::AstNode. 

Implementation notes:

The end goal here is that the extension will call a function in the API to allocate an IdLookup AstNode. The host will return a host::AggregationStageAstNode which holds a unique_ptr<LiteParsedDocumentSource> OR a unique_ptr<LiteParsedDocumentSourceIdLookup>.
LiteParsedDocumentSourceIdLookup should keep the original BSON spec, and provide a method to expose the original BSON spec.  host::AggregationStageAstNode should have a public method that allows exposing the BSON spec:

BSONObj host::AggregationStageAstNode::getIdLookupSpec() {
   return _liteParsedIdLookup->getBsonSpec();
}

This will allow us to parse the spec into a DocumentSource during expansion on the Host side of the boundary.