starting point for entity page extension API

Co-authored-by: Philipp Hugenroth <tudi2d@users.noreply.github.com>
Co-authored-by: Vincenzo Scamporlino <vinzscam@users.noreply.github.com>
Co-authored-by: Camila Belo <camilaibs@gmail.com>
Co-authored-by: Rickard Dybeck <r.dybeck@gmail.com>
Co-authored-by: Ben Lambert <ben@blam.sh>
Co-authored-by: Jack Palmer <UsainBloot@users.noreply.github.com>
Co-authored-by: Elon Jefferson <Edje-C@users.noreply.github.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

plugins/catalog/src/alpha.tsx

@@ -283,3 +283,51 @@ export default createPlugin({
     CatalogNavItem,
   ],
 });
+
+///  IMAGINE THIS IS IN A DIFFERENT PLUGIN
+
+// inside the @backstage/plugin-github-pull-requests plugin
+import {
+  createEntityCardExtension,
+  createEntityContentExtension,
+} from '@backstage/plugin-catalog-react';
+
+const githubPullRequestsPlugin = createPlugin({
+  id: 'github-pull-requests',
+  extensions: [
+    createEntityCardExtension({
+      id: 'github-pull-requests',
+      loader: () => import('./PullRequestsCard').then(m => m.PullRequestsCard),
+      entityFilter: isPullRequestsAvailable,
+    }),
+    createEntityContentExtension({
+      id: 'github-pull-requests',
+      defaultPath: 'github-pull-requests',
+      defaultTitle: 'GitHub Pull Requests',
+      loader: () =>
+        import('./PullRequestsContent').then(m => m.PullRequestsContent),
+      entityFilter: isPullRequestsAvailable,
+    }),
+  ],
+});
+
+// /deployments
+const deploymentsPlugin = createPlugin({
+  id: 'github-pull-requests',
+  extensions: [
+    createEntityCardExtension({
+      id: 'github-pull-requests',
+      attachTo: { id: 'plugin.deployments.content', input: 'cards' },
+      loader: () => import('./PullRequestsCard').then(m => m.PullRequestsCard),
+      entityFilter: isPullRequestsAvailable,
+    }),
+    createEntityContentExtension({
+      id: 'github-pull-requests',
+      defaultPath: 'github-pull-requests',
+      defaultTitle: 'GitHub Pull Requests',
+      loader: () =>
+        import('./PullRequestsContent').then(m => m.PullRequestsContent),
+      entityFilter: isPullRequestsAvailable,
+    }),
+  ],
+});