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	❌	8/16 passing
SOAK	❌	83/85 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 (b6b2639).
⚠️ Report is 12 commits behind head on dev.

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.