feat: thread context from breakpad on macos

[supervacuus]

Upstream breakpad doesn't provide the thread context to the handler callbacks (it does write it to the minidump, though). This change fills the context of the crashed thread and passes it on to the handler callback (which in turn passes it on to the on_crash and before_send callbacks).

The change keeps compatibility between upstream and our fork in both directions even though this breaks one of the public interfaces:

• our breakpad backend in the Native SDK uses the upstream interface if SENTRY_BREAKPAD_SYSTEM is defined
• our breakpad fork provides the modified interface only if compiled with the Native SDK (i.e., SENTRY_BACKEND_BREAKPAD is defined in the breakpad_client target).

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 4320b6b

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.38%. Comparing base (2ad5f0d) to head (4320b6b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1083      +/-   ##
==========================================
+ Coverage   82.36%   82.38%   +0.02%     
==========================================
  Files          53       53              
  Lines        7752     7751       -1     
  Branches     1216     1216              
==========================================
+ Hits         6385     6386       +1     
+ Misses       1255     1253       -2     
  Partials      112      112              

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[markushi]

Looks good

[markushi]

LGTM!