Implement Context propagation

[rghetia]

Implement Context propagation as per the spec.

[krnowak]

This is copy of a post from #457 about suppliers:

So this issue conflicts with the following issues in the otel-spec (as we will likely need to rework the supplier interface for allowing multi-valued keys and rework propagator interface to have an iterator over keys instead of a function for getting a fixed list of the keys the propagator uses/understands):

open-telemetry/opentelemetry-specification#433
open-telemetry/opentelemetry-specification#443
open-telemetry/opentelemetry-specification#444

[krnowak]

This is done. The supplier interface is still in works on the specification side.