Draft: Error Tracking: Collect stacktrace frames from threads

What does this MR do and why?

Sentry payloads sent from Rust contain stacktrace.frames in threads.values and not in exception.values.

See #340178 (comment 811287774) and https://gitlab.com/splattael/error-tracking-example-rust.

WIP:

• Tweak JSON schema
• Add JSON fixture rust_stacktrace_in_threads_values.json
• Add specs for service
• Add specs for stack_trace.json

Screenshots or screen recordings

stacktrace inside exception.values	stacktrace inside threads.values 🆕

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.