GroupBy on more properties in Release health session statistics API

### Problem Statement

API: https://docs.sentry.io/api/releases/retrieve-release-health-session-statistics/

As a developer I only have a small list of properties to group on
"The available groupBy conditions are project, release, environment and session.status."

I want to group the data in a way that it would make more sense for my 3th party apps I feed data to.

### Solution Brainstorm

Being able to use custom properties in the GroupBy input so we can format the data in a better way.

┆Issue is synchronized with this [Jira Improvement](https://getsentry.atlassian.net/browse/FEEDBACK-1789) by [Unito](https://www.unito.io)