⚗ [RUMF-970] experiment with buffered PerformanceObserver

[bcaudan]

Motivation

Try to retrieve largest-contentful-paint, first-input and layout-shift values that could have been emitted before SDK init.

Changes

behind 'buffered-perf' flag, create one buffered observer for each experimental type.

Testing

locally

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I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #972 (fd6d7ce) into main (4dfbddd) will decrease coverage by 0.16%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #972      +/-   ##
==========================================
- Coverage   89.08%   88.92%   -0.17%     
==========================================
  Files          83       83              
  Lines        3867     3874       +7     
  Branches      857      858       +1     
==========================================
  Hits         3445     3445              
- Misses        422      429       +7     

Impacted Files	Coverage Δ
...ages/rum-core/src/browser/performanceCollection.ts	13.86% <0.00%> (-1.04%)	⬇️

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