Add timezone and is_currently_oncall to alert group paged_users

[imtoori]

What would you like to see!

We need to show the timezone and oncall status of users in the paged_users list.

Product Area

Other

Anything else to add?

We expect the timezone to be a string (eg: "Europe/Madrid")
The is_currently_oncall field should just be a boolean

[github-actions]

The current version of Grafana OnCall, at the time this issue was opened, is v1.3.82. If your issue pertains to an older version of Grafana OnCall, please be sure to list it in the PR description. Thank you 😄!

[joeyorlando]

#3164 is related