chore(sampling): move sdk types from opentelemetry to opentelemetry_sdk

[bantonsson]

Fixes #2753

Changes

Moves the following SDK sampling types from opentelemetry::trace to opentelemetry_sdk::trace:

• SamplingDecision, SamplingResult
• These types are SDK implementation details and should be imported from opentelemetry_sdk::trace instead.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.7%. Comparing base (483b420) to head (1e5ced2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3277   +/-   ##
=====================================
  Coverage   80.7%   80.7%           
=====================================
  Files        129     129           
  Lines      23212   23212           
=====================================
  Hits       18750   18750           
  Misses      4462    4462           

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