feat(cabana): settings entries that link to an admin controller

pact.SettingsItem gains an additive Controller field, the equivalent of a
WinterCMS registerSettings 'url' => Backend::url(...) entry. A link entry
declares no Model, Form or NewModel and needs no AdminFS; start-up fails
when it combines Controller with a singleton form or a model, or names an
unregistered controller. Settings codes stay one namespace.

GET /settings lists a link entry with its controller only when the
principal passes the item's permissions and may open the controller.
Registry.Setting never returns a link entry, so the singleton settings
endpoints answer 404 for its code. SettingsEntry carries controller,
empty for singletons; the OpenAPI document, generated SPA types and
settings fixture follow, and the pact and cabana READMEs and the
settings docs describe the link.

admin/tests/fixtures/settings.json

@@ -9,6 +9,7 @@
         "category": "System",
         "keywords": [],
         "model": "Acme\\Demo\\Models\\Settings",
+        "controller": "",
         "order": 20
       },
       {
@@ -19,6 +20,7 @@
         "category": "System",
         "keywords": [],
         "model": "Acme\\Demo\\Models\\MailSettings",
+        "controller": "",
         "order": 10
       },
       {
@@ -29,6 +31,7 @@
         "category": "Appearance",
         "keywords": [],
         "model": "Acme\\Demo\\Models\\Branding",
+        "controller": "",
         "order": 5
       }
     ],