sync: add group filtering

sync: add group filtering

like for manual pulls, but persisted in the sync job config and visible
in the relevant GUI parts.

GUI is read-only for now (and defaults to no filtering on creation), as
this is a rather advanced feature that requires a complex GUI to be
user-friendly (regex-freeform, type-combobox, remote group scanning +
selector with additional freeform input).

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>