Separate baggage<->Context api from Baggage Propagator

[lalitb]

Fixes #961

Changes

As mentioned in #961, getter/setter methods for baggage on Context shouldn't be part of Baggage Propagator.

Moved

• opentelemetry::baggage::propagation::GetBaggage() to opentelemetry::baggage::GetBaggage()
• opentelemetry::baggage::propagation::SetBaggage() to opentelemetry::baggage::GetBaggage()
• opentelemetry::baggage::propagation::kBaggageHeader to opentelemetry::baggage::kBaggageHeader

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 #963 (19600ca) into main (4ae969b) will not change coverage.
The diff coverage is 90.00%.

@@           Coverage Diff           @@
##             main     #963   +/-   ##
=======================================
  Coverage   95.36%   95.36%           
=======================================
  Files         160      161    +1     
  Lines        6780     6780           
=======================================
  Hits         6465     6465           
  Misses        315      315           

Impacted Files	Coverage Δ
...pi/include/opentelemetry/baggage/baggage_context.h	75.00% <75.00%> (ø)
...telemetry/baggage/propagation/baggage_propagator.h	100.00% <100.00%> (+11.77%)	⬆️
...est/baggage/propagation/baggage_propagator_test.cc	100.00% <100.00%> (ø)