add support for OTEL_SDK_DISABLED environment variable

[tsloughter]

Resolves #573

[tsloughter]

I reordered this because I was going to put a case Disabled around the parts that don't need to run if the SDK is disabled but then didn't because its not exactly required since the supervisor simply starts no children and that causes everything to fallback to no-ops.

Undecided.. Maybe I should check if its disabled here too and not do stuff like get_resource.

[tsloughter]

Yea, I went ahead and added the check if sdk is disabled and skip the stuff below.

[codecov]

Codecov Report

Patch coverage: 92.30% and project coverage change: +0.05 🎉

Comparison is base (3bf392b) 38.05% compared to head (64fecd2) 38.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #574      +/-   ##
==========================================
+ Coverage   38.05%   38.11%   +0.05%     
==========================================
  Files          61       61              
  Lines        3592     3597       +5     
==========================================
+ Hits         1367     1371       +4     
- Misses       2225     2226       +1     

Flag	Coverage Δ
api	67.91% <ø> (ø)
elixir	18.29% <ø> (ø)
erlang	38.09% <92.30%> (+0.05%)	⬆️
exporter	8.11% <ø> (ø)
sdk	78.13% <92.30%> (+0.01%)	⬆️
zipkin	54.16% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
apps/opentelemetry/src/otel_resource_detector.erl	86.00% <ø> (ø)
...pps/opentelemetry/src/otel_tracer_provider_sup.erl	87.50% <66.66%> (-12.50%)	⬇️
apps/opentelemetry/src/opentelemetry_app.erl	100.00% <100.00%> (ø)
apps/opentelemetry/src/opentelemetry_sup.erl	100.00% <100.00%> (ø)
apps/opentelemetry/src/otel_configuration.erl	79.33% <100.00%> (ø)

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