enhancement - Add additional_info in event handler servicenow

It'd be nice if we can send additional_info using servicenow event handler, it would greatly help in handling our events properly.
https://docs.influxdata.com/kapacitor/v1.5/event_handlers/servicenow/#options

https://docs.servicenow.com/bundle/paris-it-operations-management/page/product/event-management/concept/c_EMIntegrateRequirementEvent.html
**em_event.additional_info**

Start by locating the ServiceNow event handler implementation and reviewing the linked Kapacitor and ServiceNow documentation. The work is complete when the handler accepts and sends an additional_info value with events, with coverage confirming the resulting payload.

Written by the indexing model from the issue text.