add option to only generate stubs of the API and no server files

Fix #3025

modules/swagger-codegen/src/main/resources/JavaSpringBoot/apiController.mustache

@@ -0,0 +1,35 @@
+package {{apiPackage}};
+
+import {{modelPackage}}.*;
+
+{{#imports}}import {{import}};
+{{/imports}}
+
+import io.swagger.annotations.*;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestHeader;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RequestPart;
+import org.springframework.web.multipart.MultipartFile;
+
+import java.util.List;
+
+@Controller
+{{>generatedAnnotation}}
+{{#operations}}
+public class {{classname}}Controller implements {{classname}} {
+  {{#operation}}
+    public ResponseEntity<{{>returnTypes}}> {{operationId}}({{#allParams}}{{>queryParams}}{{>pathParams}}{{>headerParams}}{{>bodyParams}}{{>formParams}}{{#hasMore}},
+      {{/hasMore}}{{/allParams}}) {
+      // do some magic!
+      return new ResponseEntity<{{>returnTypes}}>(HttpStatus.OK);
+  }
+
+  {{/operation}}
+}
+{{/operations}}