Expose a way to flag which thread is the thread to be used for grouping

When a game crashes as a result of an App Hang or deadlock, the crash itself is triggered by a watchdog type of thread, not the actual thread that was hanging or deadlocked.

This means that minidump processing marks "the incorrect" thread as crashed:true, and Sentry's UI shows the watch dog thread by default in the thread selector, and we use that for grouping too.

We need to find a way to mark which thread is to be considered crashed.

Note that this can affect customers on the Unreal Engine CRC (crash reporter client), not only minidumps captured through our SDK.

Internal ticket: https://github.com/getsentry/team-gdx/issues/99

The issue names minidump processing, Sentry's thread selector, grouping, and the Unreal Engine crash reporter client, but no files, tests, or entry points. Start by tracing how crashed:true is assigned and consumed across those paths. Done means there is a decided way to identify the thread used for grouping instead of the watchdog thread.

Written by the indexing model from the issue text.