Add UnmarshalJSON method for TraceID

[RitaGlushkova]

I had to manually pass TraceID from my Kafka Producer to Consumer for one of my projects and realised that the UnmarshalJSON method for TraceID wasn't there.

So I had to manually convert my TraceID into a string before sending it to Kafka.
So, adding this method to the source would be helpful to avoid this extra manual work.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: RitaGlushkova / name: Margarita Glushkova (bbeec46)

[codecov]

Codecov Report

Merging #3682 (a18edd6) into main (0446207) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3682     +/-   ##
=======================================
- Coverage   79.7%   79.7%   -0.1%     
=======================================
  Files        171     171             
  Lines      12673   12684     +11     
=======================================
+ Hits       10105   10112      +7     
- Misses      2355    2359      +4     
  Partials     213     213             

Impacted Files	Coverage Δ
trace/trace.go	98.3% <100.0%> (+0.1%)	⬆️
sdk/trace/batch_span_processor.go	80.2% <0.0%> (-1.8%)	⬇️

[dmathieu]

See related discussion: #1819