[WIP]: feat(appender-tracing): add experimental span attributes enrichment

[leghadjeu-christian]

Fixes #3221
Design discussion issue (if applicable) #

Changes

Add experimental experimental_span_attributes feature to enrich log records with attributes from active tracing spans.
When enabled, span attributes are automatically included in log records. Event attributes take precedence on conflicts.

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

❌ Patch coverage is 86.28571% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.8%. Comparing base (65969fd) to head (613954f).

Files with missing lines	Patch %	Lines
opentelemetry-appender-tracing/src/layer.rs	86.2%	24 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #3282    +/-   ##
======================================
  Coverage   80.7%   80.8%            
======================================
  Files        129     129            
  Lines      23210   23380   +170     
======================================
+ Hits       18748   18894   +146     
- Misses      4462    4486    +24     

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