[Watcher] Add ServiceNow to Actions

[Erikg346]

Describe the feature:
Add ServiceNow ITSM connector to watcher actions

Describe a specific use case for the feature:
We want to leverage all ServiceNow ITSM Connector in our watchers. It seems a bit frustrating to not have same connector capabilities in Watcher.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)