feat(service/linux): 🔒️ add outlinevpn group to further restrict permissions

[jyyi1]

In this PR, I updated the permission model of the "outline_proxy_controller" daemon. We will have a group named as outlinevpn, only users in this group will be able to access "outline_proxy_controller" (through UNIX socket /var/run/outline_controller). During installation, we will only add the current user to outlinevpn group. System admins are free to add more users into this group. The new permission is listed below:

srw-rw---- 1 root outlinevpn 0 Sep 22 18:41 /var/run/outline_controller

Also I replaced all shell commands to absolute paths to prevent any potential PATH pollutions.