Allow custom text propagator to be configured via application env

[ananthakumaran]

fixes #396

[codecov]

Codecov Report

Merging #408 (a4b4d90) into main (94d385d) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
- Coverage   73.27%   73.23%   -0.05%     
==========================================
  Files          53       53              
  Lines        1654     1655       +1     
==========================================
  Hits         1212     1212              
- Misses        442      443       +1     

Flag	Coverage Δ
api	68.70% <ø> (ø)
elixir	17.54% <ø> (ø)
erlang	74.65% <0.00%> (-0.05%)	⬇️
exporter	73.59% <ø> (ø)
sdk	78.09% <0.00%> (-0.10%)	⬇️
zipkin	53.84% <ø> (ø)

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Impacted Files	Coverage Δ
apps/opentelemetry/src/otel_configuration.erl	80.28% <0.00%> (-0.57%)	⬇️

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[tsloughter]

Sorry, one last thing, can you update the changelog file at the top of the repo to include this as a SDK fix.