add API server sample

[joshgav]

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.

[joshgav]

Resources to learn from and reuse:

• basic web app (add webapp & container registry/builder sample #215, add webapp from source sample #216)
• OIDC web app (started here: https://github.com/joshgav/go-sample-auth)
• Athens (https://github.com/Azure-Samples/azure-go-labs/tree/master/4-go-modules-proxy)
• https://github.com/thockin/go-build-template
• https://github.com/golang-standards/project-layout