[0.17] feat(sync): add read-write lock primitive

[jpnurmi]

Add sentry_rwlock_t backed by platform native RW locks.

#skip-changelog (internal)

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.50%. Comparing base (b4d7fb8) to head (219277a).

Additional details and impacted files

@@                     Coverage Diff                     @@
##           jpnurmi/ref/attachments    #1877      +/-   ##
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+ Coverage                    74.47%   74.50%   +0.03%     
===========================================================
  Files                          103      103              
  Lines                        26875    26920      +45     
  Branches                      4876     4880       +4     
===========================================================
+ Hits                         20014    20058      +44     
+ Misses                        5532     5531       -1     
- Partials                      1329     1331       +2     

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