[Java][Native] Fix POJO with additionalProperties to not extend HashMap

Author: JorrenH

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java

@@ -615,6 +615,13 @@ public void processOpts() {
             supportingFiles.add(new SupportingFile("JSON.mustache", invokerFolder, "JSON.java"));
             supportingFiles.add(new SupportingFile("AbstractOpenApiSchema.mustache", modelsFolder, "AbstractOpenApiSchema.java"));
             forceSerializationLibrary(SERIALIZATION_LIBRARY_JACKSON);
+
+            // Composed schemas can have the 'additionalProperties' keyword, as specified in JSON schema.
+            // In principle, this should be enabled by default for all code generators. However due to limitations
+            // in other code generators, support needs to be enabled on a case-by-case basis.
+            // The flag below should be set for all Java libraries, but the templates need to be ported
+            // one by one for each library.
+            supportsAdditionalPropertiesWithComposedSchema = true;
         } else if (libRestEasy) {
             supportingFiles.add(new SupportingFile("JSON.mustache", invokerFolder, "JSON.java"));
             forceSerializationLibrary(SERIALIZATION_LIBRARY_JACKSON);