You can use BPMN Service Task and configure it as following to invoke a REST API
Example with text input and text output:
Example with JSON input and JSON output mapping and registry based URL:
You can find implementation of org.wso2.carbon.bpmn.extensions.rest.RESTTask class here
Currently it supports GET and POST only. If you need support for other HTTP methods you have to implement custom java service task similar to REST Task implementation that supports rest of REST API functionalities.