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.

The issue names no file or test; start by locating the OpenAPI definition hosting endpoint in the C# host and compare its current key handling with host or master keys. Done means the endpoint supports anonymous access and accepts a shared API key with the requested control.

Written by the indexing model from the issue text.