[JENKINS-18114] Fixing CLI crumbs (#3019)

* [JENKINS-18114] The CLI client already asks for a crumb; we just to actually serve it, from CrumbIssuer.RestrictedApi.

* serveCliActionToAnonymousUserWithoutPermissions() was checking some stuff better checked by authentication(), and was failing since this fake client was not passing a crumb.

* Bring back CliCrumbExclusion, needed for anonymous use with no read access, and just let the client stop asking for a crumb.

* Added FullDuplexHttpServiceTest; useful to have a simple, self-contained demonstration of the HTTP Duplex transport.