Allow OpenAPI endpoint to host an anonymous definition

There is backend work required to enable this UX item: https://github.com/Azure/azure-functions-ux/issues/1221#issuecomment-373838811 to allow the OpenAPI definition in a Function App to have the same level of key control as Functions.

I would like to update that hosting endpoint to support running without an API key, and to support specifying a shared API Key, like the host or master key.