Change log for OPEN_VPN

Date Changes
2023-11-27 Enchancement
- Added support for JSON format logs.
- Mapped "HOSTNAME" to "principal.hostname".
- Mapped "PID" to "principal.process.pid".
- Mapped "CMDLINE" to "principal.process.command_line".
- Mapped "SYSTEMD_UNIT" to "additional.fields".
- Mapped "SYSTEMD_SLICE" to "additional.fields".
- Mapped "SYSTEMD_INVOCATION_ID" to "additional.fields".
- Mapped "SYSLOG_IDENTIFOER" to "additional.fields".
- Mapped "COMM" to "additional.fields".
- Added GROK pattern to map "MESSAGE"
2022-04-28 Newly created parser.