Consolidate environment variable parsing

[pellared]

Why

Address: #2219 (comment) proposed by @reyang

Partially addresses #1453.

What

Introduce EnvironmentVariableHelper and reuse it when reading the environmental variables.

Remarks

I also created open-telemetry/opentelemetry-specification#2045 when working on this issue. Thumbs up, approvals, comments, any feedback put there is welcome 😉

[codecov]

Codecov Report

Merging #2500 (6cc2ce9) into main (105c068) will increase coverage by 0.11%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main    #2500      +/-   ##
==========================================
+ Coverage   80.47%   80.59%   +0.11%     
==========================================
  Files         253      254       +1     
  Lines        8549     8507      -42     
==========================================
- Hits         6880     6856      -24     
+ Misses       1669     1651      -18     

Impacted Files	Coverage Δ
...Jaeger/Implementation/JaegerExporterEventSource.cs	50.00% <ø> (-3.85%)	⬇️
...tation/OpenTelemetryProtocolExporterEventSource.cs	68.42% <ø> (+7.13%)	⬆️
...Zipkin/Implementation/ZipkinExporterEventSource.cs	50.00% <ø> (-22.73%)	⬇️
...penTelemetry/Internal/EnvironmentVariableHelper.cs	80.00% <80.00%> (ø)
...Telemetry.Exporter.Jaeger/JaegerExporterOptions.cs	100.00% <100.00%> (+15.78%)	⬆️
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	100.00% <100.00%> (+9.09%)	⬆️
...Telemetry.Exporter.Zipkin/ZipkinExporterOptions.cs	100.00% <100.00%> (ø)
...OpenTelemetry/Resources/OtelEnvResourceDetector.cs	81.25% <100.00%> (-1.11%)	⬇️
...lemetry/Resources/OtelServiceNameEnvVarDetector.cs	72.72% <100.00%> (-2.28%)	⬇️
...metry/Trace/BatchExportActivityProcessorOptions.cs	100.00% <100.00%> (+18.18%)	⬆️
... and 6 more

[mic-max]

Looks good to me, thanks for making this change :)