Expose Group and Grouping idnumber attributes

This pull request exposes the idnumber attribute of Group and Grouping operations that was added in Moodle 2.3+.

Groups and Groupings gained an 'idnumber' attribute in Moodle 2.3 as a result of MDL-32005:
https://tracker.moodle.org/browse/MDL-32005
This 'advanced' attribute can be used to map groups and groupings to external data sources.

This improvement allows me to use the web-service to auto-populate groups based on each course-section added to a Moodle course and keep the membership of that group up to date as students add and drop the course. Without this improvement, groups can only be matched on name --  a value that teachers are more likely to accidentally edit.