[RUMF-382] prepare support for multiple feature flags

[bcaudan]

remove flag from e2e test, let's see how it goes

[codecov-io]

Codecov Report

Merging #312 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
+ Coverage   85.42%   85.56%   +0.14%     
==========================================
  Files          25       25              
  Lines        1413     1413              
  Branches      309      309              
==========================================
+ Hits         1207     1209       +2     
+ Misses        206      204       -2     

Impacted Files	Coverage Δ
packages/core/src/configuration.ts	96.55% <ø> (ø)
packages/rum/src/rum.ts	85.00% <0.00%> (+1.25%)	⬆️
packages/rum/src/rum.entry.ts	72.00% <0.00%> (+2.00%)	⬆️

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