DEV: Single admin plugin page for consistent admin plugin UX (#26024)

This commit adds new plugin show routes (`/admin/plugins/:plugin_id`) as we move
towards every plugin having a consistent UI/landing page.

As part of this, we are introducing a consistent way for plugins
to show an inner sidebar in their config page, via a new plugin
API `register_admin_config_nav_routes`

This accepts an array of links with a label/text, and an
ember route. Once this commit is merged we can start the process
of conforming other plugins to follow this pattern, as well
as supporting a single-page version of this for simpler plugins
that don't require an inner sidebar.

Part of /t/122841 internally

app/assets/javascripts/admin/addon/routes/admin-plugins-show.js

@@ -0,0 +1,16 @@
+import Route from "@ember/routing/route";
+import { inject as service } from "@ember/service";
+import { ajax } from "discourse/lib/ajax";
+import { sanitize } from "discourse/lib/text";
+import AdminPlugin from "admin/models/admin-plugin";
+
+export default class AdminPluginsShowRoute extends Route {
+  @service router;
+
+  model(params) {
+    const pluginId = sanitize(params.plugin_id).substring(0, 100);
+    return ajax(`/admin/plugins/${pluginId}.json`).then((plugin) => {
+      return AdminPlugin.create(plugin);
+    });
+  }
+}

app/assets/javascripts/admin/addon/routes/admin-route-map.js

@@ -218,6 +218,9 @@ export default function () {
       { path: "/plugins", resetNamespace: true },
       function () {
         this.route("index", { path: "/" });
+        this.route("show", { path: "/:plugin_id" }, function () {
+          this.route("settings");
+        });
       }
     );
   });