Update propagation to conform with OpenTelemetry specification

[MrAlias]

• Remove the api/propagation package in favor of its components being a part of the main otel package (part of Move OpenTelemetry API to top level go.opentelemetry.io/otel package #964)
• Remove the Propagators interface. This collides with the specifications definitions of Propagator types and is not needed given the other changes.
• Replace all HTTP* propagation interfaces with a unified TextMapPropagator and TextMapCarrier to implement the specifications definition. Instead of breaking out Injector and Extractor functionality (which is optional in the specification), this unifies everything into a single interface. The motivation for this is ...
  • It simplifies use of propagators implementing the TextMapPropagator. Instead of just specifying you want to use the propagator you need to specify you want to use if for injecting and extracting which is not the common case.
  • If a user wants to limit to only injecting or extracting they can wrap the propagator still with a custom TextMapPropagator that limits this functionality. This is not the common case so the higher barrier to use here seems appropriate.
  • If there is user demand to split out the functionality it can still be accomplished in a backwards compatible manner after the GA.
• The New function is replaced with a NewCompositeTextMapPropagator to define the functionality better and keep it in line with the specification.
• The explicit ExportHTTP and InjectHTTP function are removed. They were equivalent to method calls on the second parameter.

Resolves #1082

[codecov]

Codecov Report

Merging #1212 into master will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1212   +/-   ##
======================================
  Coverage    76.5%   76.5%           
======================================
  Files         134     133    -1     
  Lines        5801    5797    -4     
======================================
- Hits         4442    4439    -3     
+ Misses       1110    1109    -1     
  Partials      249     249           

Impacted Files	Coverage Δ
api/baggage/baggage_propagator.go	86.3% <ø> (+1.9%)	⬆️
api/global/internal/state.go	82.8% <ø> (+1.0%)	⬆️
api/global/propagation.go	0.0% <ø> (ø)
bridge/opentracing/bridge.go	40.0% <ø> (ø)
propagation.go	100.0% <ø> (ø)
propagators/trace_context.go	65.5% <ø> (ø)
... and 1 more