Add ability to see current rotation and next rotation to either schedules or on_call_shifts endpoint

[CloudTheWolf]

What would you like to see!

I would like to be able to get more information about an OnCall Schedule to display on a dashboard that is used by our support team to see who is currently on-call without having to login to Grafana.

Currently I can get the current On-Call engineer, however I would also like to display when their OnCall Shift Started and Ends, as well as who's on call next,

I could get the Next on-call Engineer by looking at the current on call engineer and locating it in the rolling_users however this would not take into account any shift swaps.

Product Area

API

Anything else to add?

No response

[github-actions]

The current version of Grafana OnCall, at the time this issue was opened, is v1.14.1. If your issue pertains to an older version of Grafana OnCall, please be sure to list it in the PR description. Thank you 😄!