feat(agent): Add sampling control based on upstream sampling decisions

[ZNeumann]

This implements the sampling spec to allow control over the agent sampling decisions based on the value in the sampled flag of an inbound traceparent header.

[newrelic-php-agent-bot]

Test Suite	Status	Result
Multiverse	✅	16/16 passing
SOAK	❌	80/85 passing

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.14%. Comparing base (22d0138) to head (6b4bad0).

Files with missing lines	Patch %	Lines
agent/php_nrini.c	91.66%	2 Missing ⚠️
axiom/nr_distributed_trace.c	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1104      +/-   ##
==========================================
+ Coverage   78.11%   78.14%   +0.02%     
==========================================
  Files         193      193              
  Lines       27991    28039      +48     
==========================================
+ Hits        21866    21910      +44     
- Misses       6125     6129       +4     

Flag	Coverage Δ
agent-for-php-7.2	78.39% <91.66%> (+0.02%)	⬆️
agent-for-php-7.3	78.41% <91.66%> (+0.02%)	⬆️
agent-for-php-7.4	78.27% <91.66%> (+0.02%)	⬆️
agent-for-php-8.0	77.47% <91.66%> (+0.02%)	⬆️
agent-for-php-8.1	77.78% <91.66%> (+0.02%)	⬆️
agent-for-php-8.2	77.39% <91.66%> (+0.02%)	⬆️
agent-for-php-8.3	77.39% <91.66%> (+0.02%)	⬆️
agent-for-php-8.4	77.41% <91.66%> (+0.02%)	⬆️

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[lavarou]

This is interesting case: traceparent flags don't have sampled set, but New Relic's tracestate has sampled flag set. Regardless, agent respects always_off. This test also shows that remote_parent_not_sampled INI is only applicable to tracestate's sampled flag.