feat: Add Envoy Propagator

[ishg]

Adding support for Envoy Propagator, i.e. injecting Span context as a binary string under a single x-ot-span-context header. This is required for applications making requests to services behind Istio. Istio currently uses the envoy propagator which defaults to looking for this header as an incoming value to decode existence of trace.

cc: @austinlparker