Adds conformance test

[bg451]

We have implemented a conformance test which validates a tracer's inject/extract carrier format. There is a conformance harness that sends a set of carriers to a client (written in python here) and reads the clients response from stdout.

The client reads a JSON blob from stdin, extracts the span context for each carrier format, then injects the span contexts into a new JSON blob and sends that over stdout.

Google Cloud Build is being used for CI, to run the cloudbuild.yaml file locally see https://cloud.google.com/cloud-build/docs/build-debug-locally