feat: add traces_sampler

[JoshuaMoelans]

Fixes #864

• Add to options
• Add sentry_traces_sampler_function typedef (like the on_crash and before_send ones
• Add samplingContext
• Add tests (also for resetting sampler->restoring default tracesSampleRate behaviour)
• Update docs (tracing and sampling). Write about callback values access outside their lifetime (cleared after transaction_start is done). (docs PR: feat(native): add traces_sampler support sentry-docs#12333)

Core changes (without tx_cxt -> tx_ctx refactor)

[github-actions]

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

Generated by 🚫 dangerJS against e1d191e

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Codecov Report

Attention: Patch coverage is 97.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.67%. Comparing base (3981ea7) to head (e1d191e).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1108      +/-   ##
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+ Coverage   82.62%   82.67%   +0.04%     
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  Files          53       53              
  Lines        7896     7930      +34     
  Branches     1234     1240       +6     
==========================================
+ Hits         6524     6556      +32     
- Misses       1261     1264       +3     
+ Partials      111      110       -1     

[supervacuus]

Great work up to now, but a few places still need to be fixed/clarified before we can merge this. A few more tests would also make sense, especially when considering changing the order of the sampling mechanisms.

[supervacuus]

Thanks for the test effort ❤️. There are only minor things left.

[supervacuus]

👍 Thx!

[markushi]

LGTM, nice!

[ShawnCZek]

From what I understand, this part of the documentation is now obsolete:

sentry-native/include/sentry.h

Lines 1738 to 1741 in 092de88

	* The second parameter is a custom Sampling Context to be used with a Traces
	* Sampler to make a more informed sampling decision. The SDK does not currently
	* support a custom Traces Sampler and this parameter is ignored for the time
	* being but needs to be provided.