mw.ForeignApi should have an option to perform anonymous requests.

Use cases:

• The target wiki accepts cross-origin requests from us, but we will not be performing any write actions and we don't need to read anything restricted, and want to avoid the overhead of CORS pre-flight request and centralauthtoken request.
• The target wiki doesn't accept cross-origin requests from us (e.g. querying from any random wiki with $wgUseInstantCommons to Commons to get information about some file) and want to move with the times and not use JSONP.