Add app hang rate function to dashboards

### Problem Statement

Add a function for [app hang rate](https://docs.sentry.io/platforms/apple/guides/ios/configuration/app-hangs/#app-hangs-v2) (iOS) to dashboards.


As a mobile user I want to centralize my experience within dashboards. This metric only exists in the project overview.

### Solution Brainstorm

_No response_

### Product Area

Dashboards

Start in the dashboards area and compare it with the existing app hang rate metric in the project overview. Trace how that metric is exposed there, then make it available as a dashboard function and verify that dashboards can display the iOS app hang rate.

Written by the indexing model from the issue text.