DocumentSourceExtensionOptimizable currently sets DocsNeededBounds to [[Unknown, Unknown] when the pipeline visitor asks for it. This means that any DocsNeededBounds information that is gathered from stages downstream of an extension is lost.

This ticket should allow extensions to declare custom DocsNeededBounds via the extensions API that the pipeline visitor is able to read. If not declared by the extension, the extension's DocsNeededBounds should default to [Unknown, Unknown] as it currently does.