[Core] dereference combine sources recursively

[BenoitZugmeyer]

Motivation

Follow up of #551 (comment)
We figured that our combine implementation was not de-referencing all sub values. This could lead to unexpected behavior when some context is stored in an event to be pushed in the future (like: pre-init addUserAction or log) and a context sub-value is mutated in the meantime.

Changes

• Change the deepMerge (and combine) implementation to always dereference their arguments
• Introduce a deepClone function to explicitly dereference a value.

Testing

Some unit tests have been added

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I have gone over the contributing documentation.

[codecov-io]

Codecov Report

Merging #560 into master will increase coverage by 0.29%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   88.23%   88.52%   +0.29%     
==========================================
  Files          39       39              
  Lines        2260     2275      +15     
  Branches      472      476       +4     
==========================================
+ Hits         1994     2014      +20     
+ Misses        266      261       -5     

Impacted Files	Coverage Δ
packages/logs/src/logs.ts	74.19% <ø> (ø)
packages/rum/src/userActionCollection.ts	98.00% <ø> (ø)
packages/core/src/utils.ts	88.14% <95.83%> (+0.43%)	⬆️
packages/logs/src/logger.ts	95.91% <100.00%> (ø)
packages/rum/src/rum.entry.ts	94.00% <100.00%> (ø)
packages/rum/src/parentContexts.ts	98.24% <0.00%> (-1.76%)	⬇️
packages/rum/src/performanceCollection.ts	68.57% <0.00%> (+1.42%)	⬆️
packages/rum/src/rum.ts	82.19% <0.00%> (+4.10%)	⬆️

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