Add Keys function to Header

[danharasymiw]

Proposed change

Add Keys function to Header.

Use case

It would be nice to be able to inject and extract tracing contexts on the Nat's Header.
The interface required for context injection/extraction with opentelemetry is defined here: TextMapCarrier

Adding a Keys function to Header would satisfy this interface.

Contribution

#1868

[grantstephens]

What would be the difference between using the TextMapCarrier and the HeaderCarrier?
Something like TextMapPropagator.Extract(ctx, propagation.HeaderCarrier(msg.Header))
Not against the proposal, but curious.