feat: add an Iterator for opentelemetry::trace::TraceState key-value pairs

[iunanua]

Changes

Implement IntoIterator for TraceState by relying into the TraceStateIter iterator.

Notes

Now the only way to extract the different entries of the TraceState is using the header() or header_delimited() methods, making it impossible to avoid allocations.
That's why providing an iterator can be valuable in certain cases where it's necessary to know the TraceState entries while avoiding serialization.

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)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.6%. Comparing base (6f75c58) to head (87cbdfc).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3164   +/-   ##
=====================================
  Coverage   80.6%   80.6%           
=====================================
  Files        126     126           
  Lines      22195   22241   +46     
=====================================
+ Hits       17898   17944   +46     
  Misses      4297    4297           

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