[OpenTracing Bridge] Support baggage-only propagation without trace context

[ChenX1993]

Resolves #3999

Similar to this in Java OpenTracing Shim: open-telemetry/opentelemetry-java#4739

[codecov]

Codecov Report

Merging #4000 (9395e52) into main (eb2b89f) will decrease coverage by 0.1%.
The diff coverage is 16.6%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4000     +/-   ##
=======================================
- Coverage   82.1%   82.0%   -0.1%     
=======================================
  Files        175     175             
  Lines      12977   12988     +11     
=======================================
  Hits       10655   10655             
- Misses      2102    2112     +10     
- Partials     220     221      +1     

Impacted Files	Coverage Δ
bridge/opentracing/bridge.go	63.9% <0.0%> (ø)
bridge/opentracing/internal/mock.go	63.9% <18.1%> (-3.2%)	⬇️

... and 1 file with indirect coverage changes