Merge source code from aspnet/Hosting into this repo

src/Hosting/Abstractions/src/EnvironmentName.cs

...ctions/src/HostingAbstractionsWebHostBuilderExtensions.cs

src/Hosting/Abstractions/src/HostingEnvironmentExtensions.cs

src/Hosting/Abstractions/src/HostingStartupAttribute.cs

src/Hosting/Abstractions/src/IApplicationLifetime.cs

src/Hosting/Abstractions/src/IHostingEnvironment.cs

src/Hosting/Abstractions/src/IHostingStartup.cs

src/Hosting/Abstractions/src/IStartup.cs

src/Hosting/Abstractions/src/IStartupFilter.cs

src/Hosting/Abstractions/src/IWebHost.cs

src/Hosting/Abstractions/src/IWebHostBuilder.cs

...ractions/src/Internal/IStartupConfigureContainerFilter.cs

...tractions/src/Internal/IStartupConfigureServicesFilter.cs

...ions/src/Microsoft.AspNetCore.Hosting.Abstractions.csproj

src/Hosting/Abstractions/src/WebHostBuilderContext.cs

src/Hosting/Abstractions/src/WebHostDefaults.cs

src/Hosting/Abstractions/src/baseline.netcore.json

src/Hosting/Hosting/src/Builder/ApplicationBuilderFactory.cs

...Hosting/Hosting/src/Builder/IApplicationBuilderFactory.cs

src/Hosting/Hosting/src/Internal/ApplicationLifetime.cs