feat: add valuable serialization support to appender-tracing

[steved]

Fixes https://github.com/open-telemetry/opentelemetry-rust/issues/2819 Design discussion issue (if applicable) N/A

Changes

Supports the valuable extension to tracing for appender-tracing. I moved the serde::Serialize extension out of appender-log into the main package in order to share it with appender-tracing, but if that's not ideal, happy to do something else.

Merge requirement checklist

• [x] CONTRIBUTING guidelines followed
• [x] 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[bot]]

Codecov Report

:x: Patch coverage is 92.32409% with 36 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 80.8%. Comparing base (86dfd4b) to head (dabc7ad). :warning: Report is 30 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry/src/logs/serde.rs	91.9%	32 Missing :warning:
opentelemetry-appender-tracing/src/layer.rs	94.2%	4 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3033     +/-   ##
=======================================
+ Coverage   80.4%   80.8%   +0.3%     
=======================================
  Files        126     127      +1     
  Lines      22237   22390    +153     
=======================================
+ Hits       17898   18110    +212     
+ Misses      4339    4280     -59     

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