move propagators to the api

[tsloughter]

It is now in the api spec and usable without the SDK: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/context/api-propagators.md#textmap-propagator

[codecov]

Codecov Report

Merging #143 (f12ecf7) into master (0e448fd) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   32.24%   32.27%   +0.03%     
==========================================
  Files          62       62              
  Lines        3145     3145              
==========================================
+ Hits         1014     1015       +1     
+ Misses       2131     2130       -1     

Flag	Coverage Δ
api	50.00% <ø> (+0.55%)	⬆️
elixir	14.28% <ø> (-3.40%)	⬇️
erlang	32.13% <ø> (+0.03%)	⬆️
exporter	18.02% <ø> (ø)
sdk	67.04% <ø> (+1.92%)	⬆️

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Impacted Files	Coverage Δ
.../opentelemetry_api/src/otel_propagator_http_b3.erl	64.51% <ø> (ø)
...opentelemetry_api/src/otel_propagator_http_w3c.erl	45.45% <ø> (ø)
apps/opentelemetry/src/otel_batch_processor.erl	73.86% <0.00%> (+1.13%)	⬆️

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