DocumentSourceInternalResultsAndMetadata is a new internal aggr stage that wraps an source (e.g. $search) which emits two separate streams: document results and metadata. At pipeline compilation time it dissolves into an Exchange with
  per-stream consumers. 

This jira adds the skeleton registration and plumbing for the stage so subsequent tickets can build on a stable base:

•  document_source_internal_results_and_metadata.idl : IDL spec with DocumentSourceResultsAndMetadataSpec and MetadataBindSpec.
• document_source_internal_results_and_metadata.h : class declaration, Metadata struct, create() / createFromBson() factory, accessors.
• document_source_internal_results_and_metadata.cpp : registration macros, constructor, create(), createFromBson(); constraints() and serialize() are MONGO_UNIMPLEMENTED_TASSERT stubs