Spec Change: Allow injectors and extractors to be separate interfaces

[dyladan]

Link to spec PR: open-telemetry/opentelemetry-specification#578

OTEP-66 originally had the propagator interface divided into separate Injectors and Extractors. In OpenTelemetry Ruby we found some advantages to this division. For example, you can have asymmetry in the formats that are injected / extracted, and there are situations where order matters. By dividing Propagators into Injectors and Extractors you can still implement the Propagator and CompositePropagator interfaces by composing individual Injectors and Extractors. This PR updates the api-propagators spec to allow for that possibility.

@mwear would you like to take a look at this and see how it can be applied?

[mwear]

This doesn't require a change to OpenTemetry JS. It allows propagators to be split into injectors and extractors if it makes sense. You will still have a Propagator (and CompositePropagator) interface, they will just be composed of individual extractors and injectors. If this is something the SIG would like to do, the spec change allows for it, but it's not required.

[legendecas]

Closing as the issue is no longer applicable.