[Palo Alto/panw.panos] Support Syslog RFC5424

[adriansr]

Panos devices can send logs in RFC5424 (newer syslog) format, but the module is hardcoded to rfc3164 (older syslog).

Currently the only workaround is to edit the module/panw/panos/config/input.yml file and enable the new RFC with the format config option for syslog input (added by #23954):

  type: syslog
+ format: 5424   
  protocol.udp:
    host: "{{.syslog_host}}:{{.syslog_port}}"

We should expose this configuration via a module setting or update the panw module to do it's own syslog parsing supporting both RFCs.

[jamiehynds]

We're currently working on RFC5424 support for the Palo Alto and all firewall integrations. Closing this issue in favour of the issue we're working on here: elastic/integrations#1878