feat: simplify active span logic

[mwear]

Which problem is this PR solving?

• Fixes wrap extracted span context in a propagate only span #1558

Short description of the changes

This PR simplifies the active span logic by only storing a span in the active context. Instead of storing plain span contexts in a context, they are wrapped in a NoopSpan on extract.

• The setExtractedSpanContext API method was updated to wrap the extracted span context in a NoopSpan before storing it in a context.
• The getExtractedSpanContext method was removed, as this should now be done by accessing the context of the current span.
• Logic in the http plugin had to be adjusted to preserve the require parent behavior.

cc: @vmarchaud - This work might be a prerequisite for #1552.

[codecov]

Codecov Report

Merging #1589 into master will increase coverage by 0.03%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #1589      +/-   ##
==========================================
+ Coverage   91.41%   91.44%   +0.03%     
==========================================
  Files         165      165              
  Lines        5055     5052       -3     
  Branches     1035     1034       -1     
==========================================
- Hits         4621     4620       -1     
+ Misses        434      432       -2     

Impacted Files	Coverage Δ
packages/opentelemetry-api/src/context/context.ts	90.00% <66.66%> (+6.66%)	⬆️
packages/opentelemetry-api/src/trace/NoopTracer.ts	100.00% <100.00%> (ø)
packages/opentelemetry-plugin-http/src/http.ts	97.28% <100.00%> (+0.01%)	⬆️
...ackages/opentelemetry-shim-opentracing/src/shim.ts	87.70% <100.00%> (ø)

[obecny]

lgtm - I think there is one typo