Extend membersOf JQL function to work with Group picker fields

Problem definition

Currently, membersOf() works only with User picker fields, which means the function checks if a certain user is a member of the group. However, the function cannot check if a certain group belongs to a parent group (it doesn't work with Group picker field types).

Suggested resolution

Extend the function (or add a new one) so that the function can check whether a group is a member of a parent group.