Provide a debusine-admin command to change the role of a group membership

Debusine now has the concept of differentiating between a "member" role for group membership and an "admin" role. The latter is able to change membership of the group itself. The debusine-admin group members ... subcommand of the administrative CLI should support adding an admin member to a group.

TODO:

• The debusine-admin group members subcommand should grow a --role option to specify the role that we are adding/removing/setting.
• The debusine-admin group members subcommand should list roles.