feat(publicip): PUBLICIP_ENABLED replaces PUBLICIP_PERIOD

- No point periodically fetch the public IP address. Could not find anything mentioning why this was added.
- Simplification of the publicip loop code
- `PUBLICIP_ENABLED` (on, off) can be set to enable or not public ip data fetching on VPN connection
- `PUBLICIP_PERIOD=0` still works to indicate to disable public ip fetching
- `PUBLICIP_PERIOD` != 0 means to enable public ip fetching
- Warnings logged when using `PUBLICIP_PERIOD`

internal/configuration/settings/errors.go

@@ -30,7 +30,6 @@ var (
 	ErrPortForwardingEnabled           = errors.New("port forwarding cannot be enabled")
 	ErrPortForwardingUserEmpty         = errors.New("port forwarding username is empty")
 	ErrPortForwardingPasswordEmpty     = errors.New("port forwarding password is empty")
-	ErrPublicIPPeriodTooShort          = errors.New("public IP address check period is too short")
 	ErrRegionNotValid                  = errors.New("the region specified is not valid")
 	ErrServerAddressNotValid           = errors.New("server listening address is not valid")
 	ErrSystemPGIDNotValid              = errors.New("process group id is not valid")