Add Custom Fields to ServiceNow ITSM connector

[adcoelho]

As a follow-up to #171320 (and similar to #176919) we want the ServiceNow ITSM connector to support an Additional fields field that allows the user to define additional values in the action in JSON format. Users, to use the ServiceNow ITSM connector, need to install the Elastic for ITSM ServiceNow application in their ServiceNow instance. The application creates an import set table and a corresponding transformation map. Kibana will import the incident to the import set table and the transformation map will transform the data before adding them to the Incident table. By convention, all fields we add are prefixed by u_. So if a user needs to set the cmdb_ci by using the Additional fields field she should set it as { "u_cmdb_ci": "my-host-name" }. Also, users can extend the import set table and the transformation map and add their custom fields. We need to educate users about this behavior by documenting it.

DoD

• Users should be able to set any field in JSON format.
• Do not support additional fields for legacy ITSM connectors.
• Tooltip and/or documentation that explains to users about the prefixing fields with u_

Example

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)