Introduce mediawiki.ForeignApi

Introduce mediawiki.ForeignApi

mw.ForeignApi is an extension of mw.Api, automatically handling
everything required to communicate with another MediaWiki wiki via
cross-origin requests (CORS).

Authentication-related MediaWiki extensions may extend it further to
ensure that the user authenticated on the current wiki will be
automatically authenticated on the foreign one. A CentralAuth
implementation is provided in I0fd05ef8b9c9db0fdb59c6cb248f364259f80456.

Bug: T66636
Change-Id: Ic20b9682d28633baa87d22e6e9fb71ce507da58d