feat(billing): introduce billing domain and service infrastructure

- **Billing Domain:**
  - Added Kotlin Multiplatform project with domain models (`TeilnehmerKonto`, `Buchung`, `BuchungsTyp`) to represent billing entities.
  - Defined serialization strategies using `InstantSerializer`.

- **Service Implementation:**
  - Introduced `BillingServiceApplication` as the main entry point for the billing service.
  - Developed `TeilnehmerKontoService` for account management and transactions.

- **Persistence Layer:**
  - Implemented Exposed repositories (`ExposedTeilnehmerKontoRepository`, `ExposedBillingRepositories`) for database interaction.
  - Added table definitions (`TeilnehmerKontoTable`, `BuchungTable`) with indexes for efficient querying.

- **Build Configuration:**
  - Setup Gradle build files for billing domain and service modules with dependencies for Kotlin, Serialization, Spring Boot, and Exposed.

- **Test Additions:**
  - Extended ZNS importer tests with new scenarios for qualification parsing

backend/services/billing/billing-service/build.gradle.kts

@@ -0,0 +1,40 @@
+plugins {
+  alias(libs.plugins.kotlinJvm)
+  alias(libs.plugins.spring.boot)
+  alias(libs.plugins.spring.dependencyManagement)
+  alias(libs.plugins.kotlinSpring)
+}
+
+springBoot {
+  mainClass.set("at.mocode.billing.service.BillingServiceApplicationKt")
+}
+
+dependencies {
+  // Interne Module
+  implementation(projects.platform.platformDependencies)
+  implementation(projects.core.coreUtils)
+  implementation(projects.backend.services.billing.billingDomain)
+
+  // Spring Boot Starters
+  implementation(libs.spring.boot.starter.web)
+  implementation(libs.spring.boot.starter.validation)
+  implementation(libs.spring.boot.starter.actuator)
+  implementation(libs.jackson.module.kotlin)
+
+  // Datenbank-Abhängigkeiten
+  implementation(libs.exposed.core)
+  implementation(libs.exposed.dao)
+  implementation(libs.exposed.jdbc)
+  implementation(libs.exposed.kotlin.datetime)
+  implementation(libs.hikari.cp)
+  runtimeOnly(libs.postgresql.driver)
+  testRuntimeOnly(libs.h2.driver)
+
+  // Testing
+  testImplementation(projects.platform.platformTesting)
+  testImplementation(libs.spring.boot.starter.test)
+}
+
+tasks.test {
+  useJUnitPlatform()
+}