Add changeset

Signed-off-by: David Festal <dfestal@redhat.com>

Author: davidfestal

.changeset/itchy-spoons-cry.md

@@ -0,0 +1,6 @@
+---
+'@backstage/plugin-app-backend': patch
+---
+
+Restore the support of external config schema in the router of the `app-backend` plugin, which was broken in release `1.26.0`.
+This support is critical for dynamic frontend plugins to have access to their config values.