[Oneway Crossdock][Part 2] Add oneway context propagation test

[peats-bond]

Adds context propagation crossdock test for oneway transports.

The test:

• client adds baggage to context
• client calls server using the context
• server calls the client back (using the same context)
• client compares baggage

[mention-bot]

@AlexAPB, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bombela, @abhinav and @breerly to be potential reviewers.

[HelloGrayson]

Noice.

[HelloGrayson]

This seems like a common need... @yurishkuro what do you think about putting something like this in OpenTracing or Jaeger?

[yurishkuro]

Why do you need access to baggage explicitly as map[string]string? I.e. why not something like

func getBaggage(ctx Context, key string) {
   if span := opentracing.SpanFromContext(ctx); span != nil {
      return span.GetBaggageItem(key)
   }
}

?