Aggregation stages (DocumentSources) registered via extensions will need a slightly different entrypoint than internally-registered stages. We should not use the REGISTER_DOCUMENT_SOURCE family of macros since those are invoked with the global initializers, but extensions will be loaded later than that. 

We should add a new function like DocumentSource::registerExtensionParser that is solely used for registering extension aggregation stages. If the "name" is a duplicate of a stage already in the parserMap, an assertion should be thrown (so that startup fails). But we should allow an exception for a $vectorSearch extension stage to safely override the original implementation.

This should all be tested with C++ unit tests.