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/build.gradle.kts

@@ -13,6 +13,7 @@ dependencies {
   // Interne Module
   implementation(projects.platform.platformDependencies)
   implementation(projects.core.coreUtils)
+  implementation(projects.core.coreDomain)
   implementation(projects.backend.services.billing.billingDomain)
 
   // Spring Boot Starters