[finished] Add schema for config

- Original schema from artoo@manjaro.org, modified for current JSON-Schema use

src/modules/finished/finished.schema.yaml

@@ -0,0 +1,11 @@
+---
+$schema: https://json-schema.org/schema#
+$id: https://calamares.io/schemas/finished
+type: object
+properties:
+    restartNowEnabled: { type: boolean, default: true }  # TODO:3.3: remove
+    restartNowChecked: { type: boolean, default: false } # TODO:3.3: remove
+    restartNowCommand: { type: string }
+    restartNowMode: { type: string, enum: [ never, user-unchecked, user-checked, always ] }
+    notifyOnFinished: { type: boolean }
+additionalProperties: false