Relocate Getter and Setter to context/propagation

[carlosalberto]

Currently Getter and Setter are located in opentelemetry.trace.propagation, even though this interface is supposed to be used everywhere for propagation (and it is indeed used in the BaggagePropagator).

Consider removing it to some context or propagation signal-independent package.

[lzchen]

Just to clarify, is this issue that this is under the opentelemetry.trace namespace and other signals could utilize propagation functionality?

[carlosalberto]

@lzchen yes ;)