feat: allow Resource transformation in TracerProviderBuilder

[bantonsson]

Changes

Adds the ability to transform a Resource in the TracerProviderBuilder so it's possible to add configuration from other sources in a safe way.

This has come up when trying to add resource configuration based on vendor specific environment variables while not overriding the resources configured by the OpenTelemetry ones.

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 (353bbb0) to head (cd60550).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3131     +/-   ##
=======================================
+ Coverage   80.6%   80.7%   +0.1%     
=======================================
  Files        126     126             
  Lines      22195   22330    +135     
=======================================
+ Hits       17898   18034    +136     
+ Misses      4297    4296      -1     

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