Non-template propagator, Implement Global Propagator, no-op propagator

[lalitb]

Fixes #540

Changes

Please provide a brief description of the changes here.

• In continuation with discussion in Add Global Propagator, Noop Propagator #644, remove the templatized propagator class, so that the propagator is not compiled against its carrier/target type, and the carrier/target type is instead passed as TextMapCarrier object in Inject() and Extract() functions. This further allows not to create global/composite propagator per carrier/target type. As an example, a global propagator once set, can be used to extract the context from (say) gRPC response, and pass the context to (say) HTTP request.

• Provide an implementation of global and No-op propagator.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #668 (3d40b82) into main (a978920) will increase coverage by 0.03%.
The diff coverage is 99.48%.

@@            Coverage Diff             @@
##             main     #668      +/-   ##
==========================================
+ Coverage   94.75%   94.78%   +0.03%     
==========================================
  Files         210      212       +2     
  Lines        9647     9707      +60     
==========================================
+ Hits         9141     9201      +60     
  Misses        506      506              

Impacted Files	Coverage Δ
.../test/trace/propagation/jaeger_propagation_test.cc	97.91% <95.23%> (+0.09%)	⬆️
...de/opentelemetry/trace/propagation/b3_propagator.h	96.49% <100.00%> (ø)
...telemetry/trace/propagation/composite_propagator.h	100.00% <100.00%> (ø)
...pentelemetry/trace/propagation/global_propagator.h	100.00% <100.00%> (ø)
...entelemetry/trace/propagation/http_trace_context.h	93.44% <100.00%> (ø)
...i/include/opentelemetry/trace/propagation/jaeger.h	97.56% <100.00%> (ø)
.../opentelemetry/trace/propagation/noop_propagator.h	100.00% <100.00%> (ø)
api/test/trace/propagation/b3_propagation_test.cc	100.00% <100.00%> (ø)
...est/trace/propagation/composite_propagator_test.cc	100.00% <100.00%> (ø)
...pi/test/trace/propagation/http_text_format_test.cc	100.00% <100.00%> (ø)
... and 2 more