make span context management public

[lalitb]

Fixes #950

Changes

( raising PR #966 again with resolved review comments because of easycla issue with one of the commits in it cc @ThomsonTan )

Move span-context helper methods from trace::propagation to trace namespace. They are now treated as being in public api namespace. It's not possible to move them to Span API ( i.e, making them static within api::Span class ) as the complete implementation of DefaultSpan is not visible/available in that class.

This is how header structure looks like:

context/context.h - contains implementation for context data-structure logic
trace/tracer.h - span context management ( implicit )
trace/context.h - span context management ( explicit )

The implicit context management are still static methods in trace::tracer class to ensure not to introduce a breaking change, and at the same time remain specs compliant.

cc @jsuereth

[codecov]

Codecov Report

Merging #967 (fade70d) into main (7b1842b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #967   +/-   ##
=======================================
  Coverage   95.36%   95.36%           
=======================================
  Files         161      161           
  Lines        6780     6780           
=======================================
  Hits         6465     6465           
  Misses        315      315           

Impacted Files	Coverage Δ
api/include/opentelemetry/trace/context.h	100.00% <100.00%> (ø)
...de/opentelemetry/trace/propagation/b3_propagator.h	93.45% <100.00%> (ø)
...entelemetry/trace/propagation/http_trace_context.h	93.66% <100.00%> (ø)
...i/include/opentelemetry/trace/propagation/jaeger.h	97.68% <100.00%> (ø)
...pi/test/trace/propagation/http_text_format_test.cc	100.00% <100.00%> (ø)
.../test/trace/propagation/jaeger_propagation_test.cc	98.15% <100.00%> (ø)