✨ [RUMF-727] introduce v2 format

[bcaudan]

Motivation

Start using v2 format and introduce format validation

Changes

• Introduce v2_format feature flag
• Duplicate types for V2
• Use dedicated events for v2
• Add validation for v2 events generated in unit tests
• Handle V2 long tasks

Testing

Automated tests

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I have gone over the contributing documentation.

[codecov-io]

Codecov Report

Merging #570 into master will decrease coverage by 0.35%.
The diff coverage is 81.60%.

@@            Coverage Diff             @@
##           master     #570      +/-   ##
==========================================
- Coverage   88.47%   88.12%   -0.36%     
==========================================
  Files          42       46       +4     
  Lines        2299     2357      +58     
  Branches      479      491      +12     
==========================================
+ Hits         2034     2077      +43     
- Misses        265      280      +15     

Impacted Files	Coverage Δ
packages/logs/src/logger.ts	95.91% <ø> (ø)
packages/rum/src/assembly.ts	100.00% <ø> (ø)
packages/rum/src/rum.entry.ts	94.00% <ø> (ø)
packages/rum/src/parentContexts.ts	85.29% <16.66%> (-14.71%)	⬇️
packages/rum/src/batch.ts	73.17% <20.00%> (-7.39%)	⬇️
packages/rum/test/formatValidation.ts	83.33% <83.33%> (ø)
packages/rum/src/resourceUtils.ts	97.26% <90.00%> (ø)
packages/core/src/errorCollection.ts	89.28% <100.00%> (ø)
packages/core/src/utils.ts	88.14% <100.00%> (ø)
packages/rum/src/assemblyV2.ts	100.00% <100.00%> (ø)
... and 10 more

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[BenoitZugmeyer]

Changes look good! I think it would have been better to rename V1 functions/types/events to add v1 in their name, instead of adding new ones with v2 (like I did on web-ui). This way, the migration cleanup would be easier. But that's fine now.