Add support for OpenAPI code generation (phase is generate-sources)

src/main/java/ch/dlmw/swisssignchallenge/controllers/HelloController.java

@@ -0,0 +1,14 @@
+package ch.dlmw.swisssignchallenge.controllers;
+
+import org.openapitools.api.HelloApi;
+import org.openapitools.model.HelloResponse;
+import org.springframework.http.ResponseEntity;
+import org.springframework.stereotype.Controller;
+
+@Controller
+public class HelloController implements HelloApi {
+    @Override
+    public ResponseEntity<HelloResponse> getHello() {
+        return ResponseEntity.ok(new HelloResponse().value("Hello, world"));
+    }
+}