Fix TraceContextPropagator behavior when trace parent flags contain…

[Kielek]

Fixes test_traceparent_trace_flags_illegal_characters from w3c test suite
Design discussion issue #

Changes

Fix TraceContextPropagator behavior when trace parent flags contains illegal characters.

Merge requirement checklist

• CONTRIBUTING guidelines followed (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

Merging #4893 (726c499) into main (f01db2b) will increase coverage by 0.04%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 726c499 differs from pull request most recent head e2cefb6. Consider uploading reports for the commit e2cefb6 to get more accurate results

@@            Coverage Diff             @@
##             main    #4893      +/-   ##
==========================================
+ Coverage   83.26%   83.30%   +0.04%     
==========================================
  Files         295      295              
  Lines       12324    12325       +1     
==========================================
+ Hits        10261    10267       +6     
+ Misses       2063     2058       -5     

Flag	Coverage Δ
unittests	83.30% <100.00%> (+0.04%)	⬆️

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

Files	Coverage Δ
....Api/Context/Propagation/TraceContextPropagator.cs	89.47% <100.00%> (+1.82%)	⬆️

... and 4 files with indirect coverage changes

[Kielek]

@open-telemetry/dotnet-approvers, I have 2 more tests fixes ready to review on separate branch. As it touches some internal implementation/decision it is worth to make separate reviews. If you think otherwise I can bring all of them in one PR. Finally it passes all W3c tests on .NET7+.

2 other fixes on the top of branch: https://github.com/Kielek/opentelemetry-dotnet/commits/tace-context-propagator-other-fixes