api: add GroupFilter(List) type

api: add GroupFilter(List) type

at the API level, this is a simple (wrapped) Vec of Strings with a
verifier function. all users should use the provided helper to get the
actual GroupFilter enum values, which can't be directly used in the API
schema because of restrictions of the api macro.

validation of the schema + parsing into the proper type uses the same fn
intentionally to avoid running out of sync, even if it means compiling
the REs twice.

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>