Add igwn-shell metapackage to bundle command interpreters
This MR adds a new igwn-shell
metapackage that just enumerates the various command interpreters (shells) from which users are allowed to choose as part of the LDG account management application.
Closes https://git.ligo.org/computing/helpdesk/-/issues/3525 (see comment).
The initial list was taken from computing/iam/LDAP/ligo-ldg-user-registration@5d915daa.