Implement an IHostingStartup

[Tratcher]

Implement IHostingStartup and add the HostingStartupAttribute so we can remove UseIISIntegration from default templates. ANCM will set ASPNETCORE_HOSTINGSTARTUPASSEMBLIES=Microsoft.AspNetCore.Server.IISIntegration and Hosting will load and add IIS.

[shirhatti]

Sanity check question. Append to ASPNETCORE_HOSTINGSTARTUPASSEMBLIES instead of set it?

[Tratcher]

Yes, ANCM should append to it.