make TracerProvider simply implement a gen_server

[tsloughter]

The API defines the functions a user calls to interact with the
TracerProvider but the handling of the gen_server calls is done
in another module. In this case the other module is in the SDK
application.

otel_tracer_server registers itself with the name
otel_tracer_provider so it acts as the default global provider.

If multiple SDK's attempt to register as the default providers
startup will fail, by design.

[codecov]

Codecov Report

Merging #298 (ed27b4a) into main (bc898b8) will decrease coverage by 0.04%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##             main     #298      +/-   ##
==========================================
- Coverage   37.25%   37.21%   -0.05%     
==========================================
  Files          46       46              
  Lines        3256     3249       -7     
==========================================
- Hits         1213     1209       -4     
+ Misses       2043     2040       -3     

Flag	Coverage Δ
api	65.07% <100.00%> (+0.20%)	⬆️
elixir	14.52% <0.00%> (+0.28%)	⬆️
erlang	37.20% <81.25%> (-0.05%)	⬇️
exporter	19.75% <ø> (ø)
sdk	77.10% <62.50%> (-0.20%)	⬇️

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Impacted Files	Coverage Δ
apps/opentelemetry/src/opentelemetry_sup.erl	100.00% <ø> (ø)
apps/opentelemetry/src/otel_tracer_server.erl	78.78% <62.50%> (-3.98%)	⬇️
...pps/opentelemetry_api/src/otel_tracer_provider.erl	83.33% <100.00%> (+13.76%)	⬆️

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