Error Tracking: Extract stacktrace from Golang exceptions

What does this MR do and why?

This MR allow ErrorTracking::StacktraceBuilder (added via !90162 (merged)) to handle stacktraces from Golang which contains a list of exceptions.

This logic was copied from Error Tracking Collector Service which perform the same transformation but on the backend side (which is currently disabled via feature flag and be removed after gitlab-org/opstrace/opstrace#1728 (closed) is finished).

Extracted from !90122 (merged).

How to set up and validate locally

See !90122 (merged)

It was verified on staging.opstracegcp.com. See !90122 (comment 999233318)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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