🐛 [RUMF-1485] Flush event when page becomes frozen

[amortemousque]

Motivation

When the browser freezes the page to preserve resources beforeunload is not triggered. Later on if the browser decides to discard the tab, the batch won’t be flushed. This PR flush the batch on freeze to prevent it.

Changes

listen to freeze event in page exit observable

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #2015 (417a832) into main (c1e1a91) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2015      +/-   ##
==========================================
+ Coverage   93.49%   93.54%   +0.04%     
==========================================
  Files         145      145              
  Lines        5507     5512       +5     
  Branches     1249     1250       +1     
==========================================
+ Hits         5149     5156       +7     
+ Misses        358      356       -2     

Impacted Files	Coverage Δ
packages/core/src/browser/pageExitObservable.ts	100.00% <100.00%> (ø)
.../src/domain/segmentCollection/segmentCollection.ts	100.00% <100.00%> (ø)
...rum-core/src/domain/contexts/foregroundContexts.ts	93.84% <0.00%> (+3.07%)	⬆️

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