perf: Baggage insert & BaggageExt::with_baggage & updated constants to latest standard

[fraillt]

Changes

Improve few things:

• BaggageExt::with_baggage now "clone" baggage, instead of iterate+collect.
• Baggage::insert_with_metadata (used everywhere) should be a bit faster, as it had redundant calculations:
  • access hashmap once (before it was twice)
  • calculate entry content length once (before it was three times, although it's so trivial that probably it's not noticeable anyway)
• updated incorrect comment on Baggage::iter

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: fraillt / name: Mindaugas Vinkelis (cc7cd67, 5201543)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.4%. Comparing base (5e47487) to head (5201543).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2284   +/-   ##
=====================================
  Coverage   79.4%   79.4%           
=====================================
  Files        123     123           
  Lines      22748   22736   -12     
=====================================
- Hits       18063   18058    -5     
+ Misses      4685    4678    -7     

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[cijothomas]

LGTM.
Would be good to add a changelog entry for the fixes in size/length validations fixes.
Will open separate issue to make Baggage spec compliant.