🔊[RUMF-408] add new session check logs

[bcaudan]

Motivation

Add checks to measure new session strategy impacts on collected sessions

Changes

Introduce a _dd_exp_s cookie to store session creation date.
Put checks behind 'new-session-checks' flag.

[codecov-io]

Codecov Report

Merging #318 into master will decrease coverage by 3.79%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
- Coverage   85.72%   81.92%   -3.80%     
==========================================
  Files          25       26       +1     
  Lines        1429     1505      +76     
  Branches      302      316      +14     
==========================================
+ Hits         1225     1233       +8     
- Misses        204      272      +68     

Impacted Files	Coverage Δ
packages/core/src/configuration.ts	96.55% <ø> (ø)
packages/core/src/internalMonitoring.ts	86.79% <0.00%> (-7.09%)	⬇️
packages/rum/src/newSessionChecks.ts	10.29% <10.29%> (ø)
packages/rum/src/rum.entry.ts	71.15% <50.00%> (-0.85%)	⬇️
packages/core/src/utils.ts	96.36% <100.00%> (+0.06%)	⬆️
packages/logs/src/loggerSession.ts	95.45% <0.00%> (-4.55%)	⬇️
packages/logs/src/logger.ts	94.36% <0.00%> (-1.41%)	⬇️

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[BenoitZugmeyer]

LGTM