Add more details when creating an incident from Backstage

[t1agob]

Is your feature request related to a problem? Please describe.
When creating an incident from the PagerDuty plugin for Backstage the only property that we are able to define is the description. This is typically not enough for on-call engineers to solve the problem.

Describe the solution you'd like
I would like to capture more information to help the on-call engineer with more context on the problem being raised, such as the severity of the event.

Describe alternatives you've considered
There isn't much else to be considered here since the UI only provides one text box. The only option would be to customise it for each configuration.