public void putExtensions(Object extensionContainer) {
        ObjectNode serialized = nodeMapper.serialize(extensionContainer).expectObjectNode();
        setExtensions(extensions.merge(serialized));
    }
Add an extension to the "extensions" object node using a POJO.

@param extensionContainer POJO to serialize and merge into extensions.
smithy-jsonschema/src/main/java/software/amazon/smithy/jsonschema/JsonSchemaConfig.java