[RUMF-617] Extract XHR and Fetch proxies

[bcaudan]

Motivation

Current proxies of XHR and fetch were managed by request collection which was handled as a singleton to avoid to proxy XHR and fetch multiple times.
With the future integration of tracing inside the sdk (cf #461), we would want to collect request with trace id for rum but not for logs.
In a npm setup this would be problematic since the request collection instance would be shared between rum and logs.

Changes

1. Extract XHR and Fetch proxies that:
   • instrument browser API only once
   • deal with request data, not with domain logic (request id, trace id, request type)
   • provide hooks to attach/retrieve info during the request life cycle
2. Use proxies in request collection and error collection
3. Move request collection to rum package

Testing

Automated tests

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I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.15419% with 11 lines in your changes missing coverage. Please review.

Project coverage is 87.50%. Comparing base (f172792) to head (6bec26c).

Files with missing lines	Patch %	Lines
packages/rum/src/rum.ts	70.58%	5 Missing ⚠️
packages/rum/src/rum.entry.ts	90.90%	4 Missing ⚠️
packages/core/src/fetchProxy.ts	97.72%	1 Missing ⚠️
packages/rum/src/requestCollection.ts	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #468      +/-   ##
==========================================
+ Coverage   87.41%   87.50%   +0.08%     
==========================================
  Files          32       34       +2     
  Lines        2034     2112      +78     
  Branches      411      416       +5     
==========================================
+ Hits         1778     1848      +70     
- Misses        256      264       +8     

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[BenoitZugmeyer]

LGTM!