Add baggage support to the opentracing shim

[bg451]

Baggage isn't truly support with the OpenTracing shim. Currently setBaggage just applies the baggage item as an attribute to the span and that's it. I think that there's a few ways to split this problem up, the first being in process baggage propagation. Basically if I set a baggage item on span A, then create a child span B and C, that baggage item should be attached to B and C. I think in process baggage can be thrown in the SpanContextShim, and whenever a new span is created check the SpanContextShim for items and apply it to the new span.

The second bit is ensuring that baggage items go into the DistributedContext so out of band baggage propagation works.

[rubenvp8510]

I would like to work on this one, if no one else if already working on it.

[vmarchaud]

@rubenvp8510 Go for it, don't hesitate to ping on the gitter room if you have any questions