Add support for source-context
Add support for source-context

Fixes: https://github.com/GoogleCloudPlatform/cloud-debug-nodejs/issues/3

gcloud cli generates a source-context.json file on deployment. This works for
Java already, but is expected to start working for Node.js as well as per some
work being done by the Cloud Debug team.

ofrobots/cloud-debug-nodejs,googleapis/cloud-debug-nodejs,googleapis/cloud-debug-nodejs,ofrobots/cloud-debug-nodejs,DominicKramer/cloud-debug-nodejs,ofrobots/cloud-debug-nodejs,matthewloring/cloud-debug-nodejs,googleapis/cloud-debug-nodejs,matthewloring/cloud-debug-nodejs,matthewloring/cloud-debug-nodejs,DominicKramer/cloud-debug-nodejs,DominicKramer/cloud-debug-nodejs