NH-2313 adjust CompositePropagator config with OTEL_PROPAGATORS

[tammy-baylis-swi]

This adjusts how customers can specify OTEL_PROPAGATORS while using NH Python. This PR is pointed at the NH-12018 branch where distro setup is split between Distro and new Configurator.

Manual test results on the lower half of the table in this doc: https://swicloud.atlassian.net/wiki/spaces/NIT/pages/2909569037/2022-04-27+to+28+propagators+combinations+testing

The below info is also on the customer documentation draft: https://swicloud.atlassian.net/wiki/spaces/NIT/pages/2867726621/NH+Python+Troubleshooting#Customizing-OTel-Propagators

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The custom-distro uses tracecontext,baggage,solarwinds_propagator propagators by default in composite for regular OTel headers and custom SolarWinds headers.

Or, the customer can specify the exact propagators they want for extract/inject behaviour in trace context propagation while using our custom-distro. This is done on customer service side as follows:

1. Install additional propagators. For example, install the Jaeger propagator with pip install opentelemetry-propagator-jaeger
2. Set OTEL_PROPAGATORS environment variable as a comma-delimited string, e.g. jaeger,tracecontext,solarwinds_propagator,dummy_external_propagator

OTEL_PROPAGATORS must be a comma-delimited string of valid propagator entry point names, which is sort-of checked by opentelemetry-python at propagator init: opentelemetry-python/init.py at main · open-telemetry/opentelemetry-python Additionally, for our custom-distro:

1. Both tracecontext and solarwinds_propagator must be included
2. tracecontext must be before solarwinds_propagator

This is because solarwinds_propagator depends on tracecontext (TraceContextTextMapPropagator) injecting tracestate header into the carrier for update before re-injection. Meanwhile the other propagators can be in any order so a customer can specify their own to run extract/inject before or after ours.

[cheempz]

LGTM, thanks for the writeup and testing @tammy-baylis-swi!