[otel] interpret OTEL_PROPAGATORS env var, and expose propagators in @opentelemetry/api

[lucacasonato]

In the future, also use them for #27919.

The default value is tracecontext,baggage, and thus the default propagators should be those.

In addition, to solve for #28082, we need to provide a Deno.telemetry.addPropagator(propagator) API, that would let users provide custom propagators at runtime.