✨ [RUMF-974] use user-agent to detect synthetics sessions

[bcaudan]

Motivation

Synthetics browser tests now add a specific value in the user agent to ease the identification, cf doc

Changes

• Check user agent instead of global variable
• Check only once

Testing

unit test
Need to check on staging

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I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #969 (4feb81d) into main (cb113a5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #969   +/-   ##
=======================================
  Coverage   89.08%   89.08%           
=======================================
  Files          83       83           
  Lines        3866     3867    +1     
  Branches      857      857           
=======================================
+ Hits         3444     3445    +1     
  Misses        422      422           

Impacted Files	Coverage Δ
packages/rum-core/src/domain/assembly.ts	97.87% <100.00%> (+0.04%)	⬆️

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[BenoitZugmeyer]

Code looks good. Could we have a small unit-test for this?