api: domains: add user/group sync API enpoint

api: domains: add user/group sync API enpoint

this api call syncs the users and groups from LDAP/AD to the
user.cfg

it also implements a 'full' mode where we first delete all
users/groups from the config and sync them again

the parameter 'enable' controls if newly synced users are 'enabled'
(if no sync parameter handles that)
the parameter 'purge' controls if ACLs get removed for users/groups
that do not exists anymore after

also add this command to pveum

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>