Adding IStartupFilter support

[jrestall]

The middleware pipeline for a tenant can now be configured using IStartupFilter, this allows libraries that use this interface as part of their initialisation to work e.g. Microsoft.AspNetCore.Mvc.Versioning, Microsoft.AspNetCore.MiddlewareAnalysis and many others.

See this comment for more info: #839 (comment)

I did investigate the two issues concerning me, the config pipeline ordering and the AutoRequestServicesStartupFilter. Both are now resolved:

• I've changed the code so that the startup filters configure the pipeline in the correct order.
• I've confirmed that AutoRequestServicesStartupFilter isn't an issue. We set the HttpContext.RequestServices property first ensuring the RequestServicesContainerMiddleware.cs doesn't set the service provider again since it checks if a provider is already set.