Provide support for Azure App Configuration

It would be nice if we could get support for azure app configuration so that in program we can retrieve connection strings and whatever else.

See the previous issue for more details on error that occurs when attempting to use Azure App Configuration package with the worker. https://github.com/Azure/azure-functions-dotnet-worker-preview/issues/27

Start by reading this issue and the linked previous issue about the Azure App Configuration package failing with the worker. Then locate the Azure Functions host and worker configuration integration points; done means the worker can retrieve connection strings and other settings from Azure App Configuration, with behavior confirmed by appropriate coverage.

Written by the indexing model from the issue text.