user: create default root user as typed struct

user: create default root user as typed struct

we added a userid attribute to the User struct, but missed that we
created the default user without that attribuet via the json! macro
which lead to a runtime panic on the deserialization

by using the struct directly, such errors will be caught by the compiler
in the future

with this change, we can remove the serde_json import here

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