🐛 [RUMF-745] fix V2 context

[bcaudan]

Motivation

in v2, customer attributes are encapsulated in a context attribute

Changes

move customer attribute to context

Testing

unit tests

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I have gone over the contributing documentation.

[codecov-io]

Codecov Report

Merging #579 into master will decrease coverage by 0.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #579      +/-   ##
==========================================
- Coverage   88.68%   88.13%   -0.56%     
==========================================
  Files          46       46              
  Lines        2359     2359              
  Branches      492      492              
==========================================
- Hits         2092     2079      -13     
- Misses        267      280      +13     

Impacted Files	Coverage Δ
packages/rum/src/domain/assemblyV2.ts	100.00% <100.00%> (ø)
packages/rum/src/domain/parentContexts.ts	85.29% <0.00%> (-11.77%)	⬇️
packages/rum/src/transport/batch.ts	73.17% <0.00%> (-9.76%)	⬇️
packages/rum/src/browser/performanceCollection.ts	67.14% <0.00%> (-1.43%)	⬇️

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