Allow for custom http middleware

Currently there is no way to address cross cutting concerns in an HttpTriggered func like, exception handling, logging and Auth/Auth. A common way to address cutting concerns in asp.net core is with middleware.

Maybe adding middleware could be done through Microsoft.Azure.Functions.Extensions.DependencyInjection.IFunctionsHostBuilder or something similar.

The issue names Microsoft.Azure.Functions.Extensions.DependencyInjection.IFunctionsHostBuilder as a possible registration point; start by tracing the HTTP-triggered function host pipeline around that extension point. Done means the supported middleware registration and execution behavior are defined for exception handling, logging, and authentication or authorization concerns.

Written by the indexing model from the issue text.