Add getActiveBaggage API

[cartermp]

Resolves #3354

[codecov]

Codecov Report

Merging #3385 (aa9abcf) into main (b29deee) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3385      +/-   ##
==========================================
- Coverage   93.29%   93.20%   -0.09%     
==========================================
  Files         247      246       -1     
  Lines        7348     7332      -16     
  Branches     1512     1507       -5     
==========================================
- Hits         6855     6834      -21     
- Misses        493      498       +5     

Impacted Files	Coverage Δ
api/src/api/propagation.ts	100.00% <100.00%> (ø)
api/src/baggage/context-helpers.ts	90.90% <100.00%> (+3.40%)	⬆️
api/karma.worker.js	0.00% <0.00%> (-100.00%)	⬇️
packages/opentelemetry-resources/karma.worker.js	0.00% <0.00%> (-100.00%)	⬇️
...ckages/opentelemetry-sdk-trace-web/karma.worker.js	0.00% <0.00%> (-100.00%)	⬇️
...kages/opentelemetry-sdk-trace-base/karma.worker.js	0.00% <0.00%> (-100.00%)	⬇️
...emetry-core/src/platform/node/RandomIdGenerator.ts	87.50% <0.00%> (-6.25%)	⬇️
.../src/platform/browser/export/BatchSpanProcessor.ts
...-trace-base/src/platform/node/RandomIdGenerator.ts	93.75% <0.00%> (+6.25%)	⬆️

[legendecas]

LGTM.

An entry needs to be added to https://github.com/open-telemetry/opentelemetry-js/blob/main/api/CHANGELOG.md.