feat: add support for notifications to feedback plugin

[drodil]

What do you want to improve?

Please add support for Backstage notifications to the feedback plugin.

What is the current behavior?

No idea if new feedback has been given unless navigating to the right page.

What will the new behavior be?

Entity owner(s) will be notified when new feedback is given for their entity. The integration should be quite straight-forward, see Backstage documentation here https://backstage.io/docs/notifications/#backend or example usage at my own plugin here: https://github.com/drodil/backstage-plugin-qeta/blob/main/plugins/qeta-backend/src/service/NotificationManager.ts

NOTE: it's possible to just pass the entityRef of the feedback target to notificationService - it will resolve the recipients internally.

[riginoommen]

Thank you for the input, @drodil. We'll definitely consider this request for the plugin and look into it. Also, if you're interested, we welcome any contributions you might want to make to the plugin.