[CONFIGURATION] Add composable sampler extensions

[shixi-li]

Fixes #4409

Changes

• Add configuration and builder interfaces for custom composable sampler extensions.
• Preserve the extension name, raw configuration node, and nesting depth during YAML parsing.
• Resolve registered composable sampler extensions through the SDK registry, with tests for parsing, registration, construction, and missing builders.

Testing

• cmake --build build --target registry_test yaml_trace_test sdk_builder_test -j4
• registry_test, yaml_trace_test, and sdk_builder_test (125 tests passed)
• clang-format --dry-run --Werror on the changed C++ files

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: shixi-li / name: shixi-li (4618bcd, ab2009b)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.69%. Comparing base (19e0244) to head (04a9ee8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4415      +/-   ##
==========================================
+ Coverage   82.65%   82.69%   +0.05%     
==========================================
  Files         503      505       +2     
  Lines       19904    19933      +29     
==========================================
+ Hits        16449    16482      +33     
+ Misses       3455     3451       -4     

Files with missing lines	Coverage Δ
...uration/composable_sampler_configuration_visitor.h	100.00% <ø> (ø)
...nfiguration/extension_composable_sampler_builder.h	100.00% <100.00%> (ø)
...ation/extension_composable_sampler_configuration.h	100.00% <100.00%> (ø)
...include/opentelemetry/sdk/configuration/registry.h	100.00% <ø> (ø)
sdk/src/configuration/configuration_parser.cc	79.90% <100.00%> (+0.21%)	⬆️
sdk/src/configuration/registry.cc	100.00% <100.00%> (ø)
sdk/src/configuration/sdk_builder.cc	58.99% <100.00%> (+0.40%)	⬆️

... and 1 file with indirect coverage changes

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[shixi-li]

/easycla

[marcalff]

LGTM

Excellent work, fits perfectly into the existing design pattern.

[dbarker]

Thanks for the contribution! Adding a custom composable sampler example in examples/configuration would be a great followup PR to show others how to use this new extension point.