feat: support multi-value key propagation extraction

[iunanua]

Changes

Modify opentelemetry::propagation::Extractor trait adding the new get_all method in order to make possible to handle keys with multiple values.
Also implement get_all method for opentelemetry_http::HeaderExtractor

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: cijothomas / name: Cijo Thomas (23d1d8b, a8d975e)
• ✅ login: iunanua / name: Igor Unanua (50e4f81, 72f64fb, 8dcbb69)

[codecov]

Codecov Report

Attention: Patch coverage is 91.48936% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.0%. Comparing base (7b3db0b) to head (50e4f81).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-http/src/lib.rs	92.5%	2 Missing ⚠️
opentelemetry/src/propagation/mod.rs	90.0%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3008   +/-   ##
=====================================
  Coverage   81.0%   81.0%           
=====================================
  Files        126     126           
  Lines      24821   24868   +47     
=====================================
+ Hits       20109   20152   +43     
- Misses      4712    4716    +4     

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