Merge pull request #4483 from andrewthauer/feat/custom-icons

feat(core-api): support custom app icons

.changeset/eight-doors-matter.md

@@ -0,0 +1,19 @@
+---
+'@backstage/core-api': patch
+'@backstage/plugin-catalog': patch
+---
+
+Minor refactoring of BackstageApp.getSystemIcons to support custom registered
+icons. Custom Icons can be added using:
+
+```tsx
+import AlarmIcon from '@material-ui/icons/Alarm';
+import MyPersonIcon from './MyPerson';
+
+const app = createApp({
+  icons: {
+    user: MyPersonIcon // override system icon
+    alert: AlarmIcon, // Custom icon
+  },
+});
+```