Add support for Haystack trace propagation

[ldoin]

I'm working on a service receiving requests that come from an external system using Haystack as a tracing system. To maintain complete traces between the two systems and still use datadog internally It would be great to have support for Haystack.

I have seen that the java agent has support for it (see https://github.com/DataDog/dd-trace-java/pull/895) and was wondering if you would be open to a contribution to add this for the cpp tracer ?