Add support for result, attachments, context to core_xapi library

The basic xAPI library covers actor, verb, object parts of statement but uses generic items as attachment, result or context.

In case some early adopters uses that other statement properties will difficult the future grow of that library. By implementing basic classes for handling those properties future improvements will be compatible with that initial implementations also and will prevent Moodle to use a long term deprecation protocol and release 3.9 with a more complete xAPI solution.

Link to the xAPI specification: https://github.com/adlnet/xAPI-Spec/blob/master/xAPI-Data.md