Unique in the collection, for example session:<id>.
The caller's kind of source, for example session or document.
The groups that may see it; a principal sees it when in at least one.
Optional folderOptional tagsOptional titleOptional metadataCopied onto every passage. The keys the index writes come from the fields above alone: a folderId or title
here, or in a passage's own metadata, is dropped.
Optional metadata?: Record<string, MetadataValue>
One source and its passages.