tracestate + baggage todos

[brettmc]

remove baggage TODOs which have since been done.
implement TODOs from tracestate to log warnings.
tidy up tracestate code, simplify some const names.

[codecov]

Codecov Report

Merging #894 (31c30c0) into main (602ea85) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #894   +/-   ##
=========================================
  Coverage     80.90%   80.90%           
+ Complexity     1906     1905    -1     
=========================================
  Files           252      252           
  Lines          4932     4932           
=========================================
  Hits           3990     3990           
  Misses          942      942           

Flag	Coverage Δ
7.4	80.35% <100.00%> (ø)
8.0	80.88% <100.00%> (ø)
8.1	81.02% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/API/Trace/TraceState.php	100.00% <100.00%> (ø)

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