Once all DocumentSource conversion goes through the new StageParam -> DocumentSource mapping registry, we can implement a mapping from extension's LiteParsed to DocumentSource translation.

We should implement ExtensionStageParams with the astNode and then call astNode->bind() to map to the LogicalStage held by the DocumentSource.

josh.siegel@mongodb.com and I believe this should mean that we can remove DocumentSourceExtensionExpandable so that expansion always happens in LiteParsed, and we just need a single DocumentSourceExtension implementation.