feat(billing): implement REST API, database config, and tests for billing service

- **REST API:** Added `BillingController` with endpoints for managing participant accounts and transactions, including history retrieval.
- **Database Configuration:** Introduced `BillingDatabaseConfiguration` to initialize database schema using Exposed.
- **Testing:** Added integration tests for `TeilnehmerKontoService` using H2 in-memory database.

backend/services/billing/billing-service/src/test/resources/application-test.yml

@@ -0,0 +1,9 @@
+spring:
+  datasource:
+    url: jdbc:h2:mem:billing_test;DB_CLOSE_DELAY=-1
+    driver-class-name: org.h2.Driver
+    username: sa
+    password: ""
+  h2:
+    console:
+      enabled: true