[otlp] Implement Span and SpanLink flags

[stevejgordon]

Changes

Implementation to populate the flags field when exporting a Span or SpanLink through OTLP, including the W3C trace flags and parent_is_remote flags specification.

See:

• Proto definition
• Indicate if a span's parent or link is remote using 2 bit flag as described in OTEP 0182 opentelemetry-proto#484
• Java implementation

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 85.58%. Comparing base (6250307) to head (42e4362).
Report is 189 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5563      +/-   ##
==========================================
+ Coverage   83.38%   85.58%   +2.19%     
==========================================
  Files         297      289       -8     
  Lines       12531    12500      -31     
==========================================
+ Hits        10449    10698     +249     
+ Misses       2082     1802     -280     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	85.30% <100.00%> (?)
unittests-Solution-Stable	85.54% <100.00%> (?)

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

Files	Coverage Δ
...metryProtocol/Implementation/ActivityExtensions.cs	96.08% <100.00%> (+0.15%)	⬆️

... and 79 files with indirect coverage changes

[CodeBlanch]

LGTM