✨[RUMF-430] new session cookie format

[bcaudan]

• new _dd_s cookie to store all session info
• maintain _dd, _dd_l and _dd_r cookies to allow rollback without compatibility issue
• add migration to not break active sessions during the new format deployment

[codecov-io]

Codecov Report

Merging #337 into master will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #337      +/-   ##
==========================================
+ Coverage   85.53%   85.87%   +0.33%     
==========================================
  Files          26       26              
  Lines        1466     1487      +21     
  Branches      312      312              
==========================================
+ Hits         1254     1277      +23     
+ Misses        212      210       -2     

Impacted Files	Coverage Δ
packages/core/src/oldCookiesMigration.ts	100.00% <100.00%> (ø)
packages/core/src/sessionManagement.ts	100.00% <100.00%> (ø)
packages/core/src/utils.ts	96.42% <100.00%> (+0.06%)	⬆️
packages/logs/src/loggerSession.ts	100.00% <100.00%> (+4.54%)	⬆️
packages/rum/src/rumSession.ts	100.00% <100.00%> (ø)
packages/logs/src/logger.ts	95.77% <0.00%> (+1.40%)	⬆️

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[BenoitZugmeyer]

LGTM