Add allowance service and create get endpoint for allowance

frontend/allowance-planner-v2/src/app/models/allowance.ts

@@ -0,0 +1,9 @@
+export interface Allowance {
+    id: number;
+    name: string;
+    target: number;
+    // Current allowance value
+    progress: number;
+    // Can be any positive number (backend checks for number relative to each other)
+    weight: number;
+}