⚗️[RUM-2889] Bootstrap custom vital APIs

[bcaudan]

Motivation

Initial PR to provide APIs to track custom duration as vital events

Changes

Behind custom_vitals flag, add APIs:

DD_RUM.startDurationVital('foo')
// do some stuff
DD_RUM.stopDurationVital('foo')

and support the new event type across the code base:

Note: First PR with the simplest API, more features in following PRs (customer context, rate limiter, ...)

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (2f0fbbc) 92.78% compared to head (5752874) 92.78%.

Files	Patch %	Lines
packages/rum-core/test/fixtures.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2591      +/-   ##
==========================================
- Coverage   92.78%   92.78%   -0.01%     
==========================================
  Files         235      236       +1     
  Lines        6794     6820      +26     
  Branches     1489     1492       +3     
==========================================
+ Hits         6304     6328      +24     
- Misses        490      492       +2     

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