[Api] GetTracer supports schema URL

[Kielek]

Fixes #2417
API does not provide GetMeter methods. It is recommended to utilize Meter constructor from System.Diagnostics.DiagnosticSource.

Changes

Public API changes based on https://github.com/dotnet/roslyn/blob/main/docs/Adding%20Optional%20Parameters%20in%20Public%20API.md
Required by https://github.com/open-telemetry/opentelemetry-specification/pull/1666/files

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.82%. Comparing base (4e2c575) to head (294e65e).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6736      +/-   ##
==========================================
+ Coverage   86.78%   86.82%   +0.04%     
==========================================
  Files         258      258              
  Lines       11990    12001      +11     
==========================================
+ Hits        10405    10420      +15     
+ Misses       1585     1581       -4     

Flag	Coverage Δ
unittests-Project-Experimental	86.45% <100.00%> (-0.26%)	⬇️
unittests-Project-Stable	86.71% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry.Api/Trace/TracerProvider.cs	92.92% <100.00%> (+0.76%)	⬆️

... and 1 file with indirect coverage changes