add the namespace property to Subsegment

In order to properly trace custom remote segments in XRay the namespace
needs to be set to "remote". This allows
TypeScript clients to correctly set
the remote namespace on the SubSegment

Author: Alex McKenzie

packages/core/lib/segments/attributes/subsegment.d.ts

@@ -9,6 +9,7 @@ declare class Subsegment {
   subsegments?: Array<Subsegment>;
   parent: SegmentLike;
   segment: Segment;
+  namespace?: string;
 
   constructor(name: string);