Who's on Call widget for main dashboard, showing who's on call for multiple services

[sploschee]

Is your feature request related to a problem? Please describe.
As an engineer, I can be named on multiple escalation policies. When I'm not on call, it can be difficult to get a summary view of who's on call for all services that I'm concerned with.

Describe the solution you'd like
For the main Backstage dashboard, I would like a widget showing who's on call for all the services where the user logged-in is named on the escalation policy.

Describe alternatives you've considered
An alternative, could be a widget that showed who's on call for a customised list of services (customisable by the user logged into Backstage). It would be great if the user could favourite services for display. This alternative would be useful to a wider range of people as not all Backstage users are engineers or on-call, but many are consumers of services and would appreciate a quick summary and link to frequently used services.

[t1agob]

Thank you for your first feature request @sploschee!

I will associate this to another open issue (#45) that is in a way trying to solve a similar problem. Although it is not exactly the same, I believe they can be potentially solved following the same approach.