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/settings_test.go

@@ -78,7 +78,6 @@ func Test_Settings_String(t *testing.T) {
 |   ├── Process UID: 1000
 |   └── Process GID: 1000
 ├── Public IP settings:
-|   ├── Fetching: every 12h0m0s
 |   ├── IP file path: /tmp/gluetun/ip
 |   └── Public IP data API: ipinfo
 └── Version settings: