Minmize distro API

[pellared]

Why

One or more configuration variables may be needed to properly configure GDI repositories. Components that can be configured with environment variables MUST support configuration of these variables using environment variables. Any component that cannot be configured with environment variables MUST support configuration of these variables using an alternate method. Any component MAY support configuration of these variables by additional methods.

Supporting only a single configuration method is simplifying the codebase and adoption.

From: https://github.com/signalfx/gdi-specification/blob/main/specification/configuration.md

What

• Minimize github.com/signalfx/splunk-otel-go/distro API to
  contain only necessary option functions.
  • Remove WithAccessToken function,
    use SPLUNK_ACCESS_TOKEN environment variable instead.
  • Remove WithEndpoint function,
    use one of the
    OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_TRACES_ENDPOINT, OTEL_EXPORTER_JAEGER_ENDPOINT
    environment variables instead.
  • Remove WithPropagator function,
    use OTEL_PROPAGATORS environment variable instead.
  • Remove WithTraceExporter function,
    use OTEL_TRACES_EXPORTER environment variable instead.
• Remove most of distro/README.md as I find our official doc in better shape. I plan to create a docs/advanced-config.go based on our official doc in the following days. It should be easier to keep both of them in sync that way and the structure will be similar to our other distros.

Testing

Scenario	Trace
default config	✅
SPLUNK_REALM=us0	✅
OTEL_TRACES_EXPORTER=jaeger-thrift-splunk SPLUNK_REALM=us0	✅
OTEL_TRACES_EXPORTER=jaeger-thrift-splunk OTEL_EXPORTER_JAEGER_ENDPOINT=http://localhost:14268/api/traces	✅

[codecov-commenter]

Codecov Report

Merging #941 (49be2db) into main (f6edc90) will decrease coverage by 0.48%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #941      +/-   ##
==========================================
- Coverage   79.80%   79.31%   -0.49%     
==========================================
  Files          56       56              
  Lines        2832     2780      -52     
==========================================
- Hits         2260     2205      -55     
- Misses        525      527       +2     
- Partials       47       48       +1     

Flag	Coverage Δ
Linux	78.95% <100.00%> (-0.50%)	⬇️
Windows	75.44% <100.00%> (-0.62%)	⬇️
macOS	75.21% <100.00%> (-0.57%)	⬇️

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

Impacted Files	Coverage Δ
distro/config.go	100.00% <ø> (ø)
distro/exporter.go	95.71% <100.00%> (-4.29%)	⬇️

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[MrAlias]

LGTM. Minor docs concerns only.