Extract FunctionTraitsSharedProxy
Extract FunctionTraitsSharedProxy

Summary: [Folly] Extract `FunctionTraitsSharedProxy`, deduplicating four nearly identical implementations.

Differential Revision: D13461506

fbshipit-source-id: 2927dbe1629024cf778301c509b82711940a8099

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly