feat: add on ending span processor functionality

[majanjua-amzn]

Fixes: N/A
Design discussion issue: open-telemetry/opentelemetry-specification#4024

Changes

Changes based loosely on the same changes made to opentelemetry-java: open-telemetry/opentelemetry-java#6367

This change creates a new ExtendedBaseProcessor that allows users to implement onEnding functionality to their processor, as per the spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#onending

Specific changes:

• Changed CompositeProcessor to be extended from the new ExtendedBaseProcessor to support processors that have OnEnding
• Created new ExtendedBaseProcessor as the place where this functionality is added, so as to not break any current experience of extending the normal BaseProcessor
• Updated TestActivityProcessor to extend the new OnEnding method while being agnostic to it for tests that don't need it
• Updated unit tests to verify the running order of the span lifecycle is correct and includes the OnEnding invocation

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

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.71%. Comparing base (cb8d9e9) to head (e108c44).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/OpenTelemetry/ExtendedBaseProcessor.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6617   +/-   ##
=======================================
  Coverage   86.70%   86.71%           
=======================================
  Files         258      259    +1     
  Lines       11955    11970   +15     
=======================================
+ Hits        10366    10380   +14     
- Misses       1589     1590    +1     

Flag	Coverage Δ
unittests-Project-Experimental	86.66% <93.33%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
src/OpenTelemetry/CompositeProcessor.cs	95.83% <100.00%> (+0.24%)	⬆️
src/OpenTelemetry/Trace/TracerProviderSdk.cs	99.38% <100.00%> (+0.01%)	⬆️
src/OpenTelemetry/ExtendedBaseProcessor.cs	0.00% <0.00%> (ø)

[Kielek]

This feature is still in development phase. It means that we cannot include it into stable releases for this package.
We have possibility to make it happen for unstable versions.

You can make full search for OTEL1000 in this repository. In this way you can find an example how to implement conditional compilation + required documentation.
OTEL1005 is the first available code at the moment.

Another thing for consideration: double check if it is not breaking change on binary level when you are introducing new class to the inheritance.