Change log for PAN_CORTEX_XDR_EVENTS

Date Changes
2023-12-15 Enhancement:
- Mapped "event_timestamp" to "metadata.event_timestamp".
- When "event_type" is "5/6" and "action_remote_ip", "action_local_ip", "agent_hostname" are null, then mapped "metadata.event_type" to "GENERIC_EVENT."
2023-02-01 Newly created parser.