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-domain/build.gradle.kts

@@ -0,0 +1,26 @@
+plugins {
+  alias(libs.plugins.kotlinMultiplatform)
+  alias(libs.plugins.kotlinSerialization)
+}
+
+kotlin {
+  jvm()
+  js(IR) {
+    browser()
+  }
+
+  sourceSets {
+    val commonMain by getting {
+      dependencies {
+        implementation(projects.core.coreDomain)
+        implementation(projects.core.coreUtils)
+        implementation(libs.kotlinx.datetime)
+      }
+    }
+    val commonTest by getting {
+      dependencies {
+        implementation(kotlin("test"))
+      }
+    }
+  }
+}