feat(appender-tracing): re-gate span attribute enrichment behind experimental_span_attributes feature

[cijothomas]

Re-introduces the experimental_span_attributes cargo feature flag and gates the API added in #3482 (TracingSpanAttributes, with_tracing_span_attributes, related Layer impls and tests) behind it.

No logic changes — just feature gating, so we have time to gather feedback and stabilize the API after this release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.8%. Comparing base (f744509) to head (067e7d7).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3505   +/-   ##
=====================================
  Coverage   82.8%   82.8%           
=====================================
  Files        130     130           
  Lines      27276   27283    +7     
=====================================
+ Hits       22609   22616    +7     
  Misses      4667    4667           

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