TextMapPropagator Getter.keys

[dyladan]

Our current getter and setter arguments are just functions. They should be changed to be interfaces, and the getter should include a keys function. Changing them both to be interfaces now will make it easier to add additional required operations in the future.

https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/context/api-propagators.md#keys