add API server sample

Create a sample runnable API server. Include a resource manager template and a clickable link in the README to deploy with that template, learning from items in https://github.com/Azure/azure-quickstart-templates.

API sample requirements:

* Compiled artifacts should be packaged as a container image stored in Azure Container Registry.
* Container image should run on Azure Web Apps or Kubernetes Service.
* API models should be persisted in CosmosDB via the MongoDB API.
* Instrument the server with OpenCensus and gather traces in App Insights.

It would also be nice if the API supported authorization via Azure AD.

Start by reviewing the repository's existing Go samples and the Azure quickstart templates linked in the issue. Define the sample around the listed requirements: a container image in Azure Container Registry, deployment to Web Apps or Kubernetes Service, CosmosDB through MongoDB API, OpenCensus traces in App Insights, and optionally Azure AD authorization. Done means the runnable sample, resource manager template, and README deployment link cover the agreed scope.

Written by the indexing model from the issue text.