Add support for creating a PR in source repo when updating mapping

[t1agob]

Is your feature request related to a problem? Please describe.
The new service mapping feature allows users to map existing PagerDuty services to Backstage entities and provides a simple way to visualize if the configuration is in sync. But doesn't provide an easy way to push the changes to code.

Describe the solution you'd like
I would like to be able to create a PR (automatically or not) to keep my code in sync with the mappings I define in Backstage.

Describe alternatives you've considered
Currently the only option is to create a PR myself which forces me to go to each service individually and create a similar PR across repos.