Improved package support in Team Build

This post introduces major improvements to the NuGet, npm, and Maven build tasks in Team Build. For NuGet, it consolidates the Installer/Restore, Packager, Publisher, and Command tasks into a single 'NuGet' task and adds a 'NuGet Tool Installer' task to control the NuGet version. It also introduces feed pickers for restore/push commands and credentials for external feeds. For npm, it focuses on better support for authenticated feeds and adds a 'command' selector. For Maven, it adds support for authenticated feeds within the same VSTS account or TFS collection. The .NET Core task also sees improvements for NuGet operations and a new '.NET Core Tool Installer' task.