feat: allow specifying app level feature flags

application feature flags can be defined in the application
creation. see docs/plugins/feature-flags.md for reference.

closes #15553

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

.changeset/moody-eggs-smash.md

@@ -0,0 +1,7 @@
+---
+'@backstage/app-defaults': minor
+'@backstage/core-plugin-api': minor
+'@backstage/core-app-api': minor
+---
+
+Allow defining application level feature flags. See [Feature Flags documentation](/docs/plugins/feature-flags.md) for reference.

.changeset/rare-cooks-peel.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-user-settings': patch
+---
+
+Feature flags now accept a description property.