Add an option to disable printing the timestamp. This is useful to not
have duplicate timestamps, when stderr is piped into a logging system
that adds timestamps on its own. For example with systemd's journald:
$ journalctl -u open5gs-smfd
Apr 10 13:25:18 hostname open5gs-smfd[1582]: 04/10 13:25:18.274: [app] INFO: Configuration: '/etc/open5gs/smf.yaml' (../lib/app/ogs-init.c:130)
Configuration change:
```
<OLD Format>
logger:
file: /var/log/open5gs/smf.log
<NEW Format>
logger:
file:
path: /var/log/open5gs/smf.log
```
Example config, to have no timestamps on stderr:
```
logger:
default:
timestamp: false
file:
path: /var/log/open5gs/smf.log
timestamp: true
```
Oliver Smith
Sukchan Lee