posix: add getentropy() based RNG

[gdamore]

XPG8 defines getentropy() as the only good source for random numbers. However, real world use a bit more nuanced. On BSD systems, we would prefer to use arc4random as it avoids unnecessary system calls. On Linux however, getentropy is implemented in terms of getrandom, and should be used directly when available.

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.66%. Comparing base (4ae376e) to head (773950e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/platform/posix/posix_rand_getentropy.c	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1880      +/-   ##
==========================================
+ Coverage   78.30%   79.66%   +1.35%     
==========================================
  Files          93       95       +2     
  Lines       20691    21636     +945     
==========================================
+ Hits        16203    17237    +1034     
+ Misses       4488     4399      -89     

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