Provide in-memory sink impl as fallback.

account-service-provider/src/main/java/de/sample/schulung/accounts/domain/sink/CustomerSinkAutoConfiguration.java

@@ -0,0 +1,16 @@
+package de.sample.schulung.accounts.domain.sink;
+
+import org.springframework.boot.autoconfigure.AutoConfiguration;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.context.annotation.Bean;
+
+@AutoConfiguration
+public class CustomerSinkAutoConfiguration {
+
+  @Bean
+  @ConditionalOnMissingBean
+  CustomersSink inMemoryCustomersSink() {
+    return new CustomersSinkInMemoryImpl();
+  }
+
+}