If I interpret the docs correctly, it is currently not possible to call the Jira Software API from a Forge app.
I have been playing around with a sample app for a while trying to call
api.asApp().requestJira("/rest/devinfo...") but I got to
403 - OAuth 2.0 is not enabled for this method.
Can anybody confirm that the Jira Software API is currently not accessible for Forge apps? Any idea when we could expect this?