fix(api): add public 'fields' function to api.propagator

[blumamir]

Which problem is this PR solving?

• fixes api.propagation.fields should be public #1741

Short description of the changes

• add fields function to api.propagator, which calls fields function on the global propagator and return the result

[codecov]

Codecov Report

Merging #1813 (c89ce15) into master (139d9db) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1813      +/-   ##
==========================================
+ Coverage   92.44%   92.51%   +0.07%     
==========================================
  Files         171      173       +2     
  Lines        5913     6038     +125     
  Branches     1264     1284      +20     
==========================================
+ Hits         5466     5586     +120     
- Misses        447      452       +5     

Impacted Files	Coverage Δ
...y-api/src/context/propagation/TextMapPropagator.ts	14.28% <ø> (ø)
packages/opentelemetry-api/src/api/propagation.ts	93.33% <100.00%> (+0.47%)	⬆️
...s/opentelemetry-instrumentation-fetch/src/fetch.ts	95.86% <0.00%> (ø)
...-instrumentation-fetch/src/enums/AttributeNames.ts	100.00% <0.00%> (ø)