Only eagerly process main/primary stack trace

Related to #51517 and https://github.com/getsentry/sentry/issues/51518. And also https://github.com/getsentry/rfcs/pull/106 eventually.

We want to only do the minimum amount of work necessary to properly group an event, and to display its primary / main thread stack trace.

We assume that the number of users that actually look at a background thread stack trace is very low. It should be possible to skip a ton of work, and do that work on-demand when a user switches to a background thread in the main event UI.

This work includes:
- Symbolication and source context application.
- Enhancement rules that categorize the stack frames and influence how it is displayed in the UI.

Apart from avoiding a ton of processing resources on things that users are extremely unlikely to ever look at, we would also save on storage costs as those parts of the event payload would only be backfilled on demand.

Doing this however is a high-effort initiative as it touches on a lot of places in the whole pipeline. Its success is also dependent on:
- Availability of minidumps, as they are needed to retroactively stack walk background threads if that is not done eagerly.
- Availability of debug symbols and source maps. As we are currently thinking about / planning for newer uploads to artifact bundles overwriting previously uploaded files, this means that *older* events might not resolve the same with *newer* debug files.