Support the OTEL_PROPAGATORS environment variable

[MrAlias]

Add support for the OTEL_PROPAGATORS environment variable as defined in the specification.

This will require designing and ratifying a plan to support loading all the propagators we need without having the SDK import them all. As was mentioned here we could look at the https://pkg.go.dev/plugin package. Or, as was suggested in a SIG meeting, look into maybe some registration method similar to the database/sql package.

Part of #1698

[MrAlias]

It might also be the case that we do not want to ultimately support this. Part of this issue is going to be determining the feasibility and appropriateness of this configuration option.

[pellared]

look into maybe some registration method similar to the database/sql package

I see it as the best approach as

1. it does not force any dependencies
2. it gives the possibility to select the propagators that are supported by the application
3. it allows also building a convenience module/package that will contain the propagators that should be supported by the spec (tracecontext and baggage)
4. I find it good when we can reuse a pattern used in the standard library

https://pkg.go.dev/plugin is rather a no-go as it does not work on all platforms. I find it also not suitable for this use case.

FYI this is something I plan to do in .NET SDK: open-telemetry/opentelemetry-dotnet#1453 (comment)