Ability to include changelog information on pipeline script

Currently there is not opportunity to include changes (github changelog) in messages from pipeline script, but this option is available in freestyle projects (Post-build Actions -> Slack Notifications -> Advanced -> Notification message includes).

Compare pipeline-script notifications with the existing freestyle path at Post-build Actions → Slack Notifications → Advanced → Notification message includes. Trace the pipeline notification entry point and determine how changelog information should be exposed there. Done means pipeline messages can include the GitHub changelog option available to freestyle projects.

Written by the indexing model from the issue text.