[Extensions] Update BaggageActivityProcessor to use baggage key predicate

[MikeGoldsmith]

Fixes #1695

Changes

• Updates the BaggageActivityProcessor to require a predicate as a constructor parameter
• Use the predicate function to determine which baggage entries are added to the new activity
• Add a public static predicate that allows all baggage entries
• Update README with examples of using the all baggage and a custom predicates

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

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.71%. Comparing base (71655ce) to head (761f4cf).
Report is 333 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1816       +/-   ##
===========================================
+ Coverage   73.91%   85.71%   +11.79%     
===========================================
  Files         267        8      -259     
  Lines        9615      140     -9475     
===========================================
- Hits         7107      120     -6987     
+ Misses       2508       20     -2488     

Flag	Coverage Δ
unittests-Extensions	85.71% <100.00%> (?)

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

Files	Coverage Δ
...ons/Internal/OpenTelemetryExtensionsEventSource.cs	53.84% <100.00%> (+8.39%)	⬆️
...metry.Extensions/Trace/BaggageActivityProcessor.cs	100.00% <100.00%> (ø)
...etry.Extensions/TracerProviderBuilderExtensions.cs	94.44% <100.00%> (+27.77%)	⬆️

... and 265 files with indirect coverage changes

[MikeGoldsmith]

Ping @open-telemetry/dotnet-contrib-approvers @open-telemetry/dotnet-contrib-maintainers - please can you take a look?

[MikeGoldsmith]

Thanks for the feedback @CodeBlanch - I've made some updates, please take another look.