Implement opt-in PERFLAB_PLACE_OBJECT_CACHE_DROPIN for Server-Timing object-cache.php placement

[b1ink0]

Summary

Fixes #1633

Relevant technical choices

Introduce PERFLAB_PLACE_OBJECT_CACHE_DROPIN constant to opt-in to placing the Server-Timing object-cache.php drop-in.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.51%. Comparing base (ff8b62b) to head (d228f28).

Files with missing lines	Patch %	Lines
plugins/performance-lab/load.php	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #1996      +/-   ##
==========================================
- Coverage   72.51%   72.51%   -0.01%     
==========================================
  Files          85       85              
  Lines        7023     7025       +2     
==========================================
+ Hits         5093     5094       +1     
- Misses       1930     1931       +1     

Flag	Coverage Δ
multisite	72.51% <50.00%> (-0.01%)	⬇️
single	40.34% <50.00%> (+<0.01%)	⬆️

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