[Bootstrapper] Fetch information from dotnet feeds

[edvilme]

#348

Fetch information from dotnet feeds for search and install command. Added logic to find global.json file and fetch version information. Added logic for downloading files.

Search

Added search command to list all available sdk installs.

bootstrapper search [<channel>] [--allow-previews]

If no channel is provided, it will list all of them by default.

Install

Added install command to install all assets from a specified version.

bootstrapper install [<version>] [--allow-previews]

If no version is provided, it will look up the nearest global.json file and install the version specified there.

Usage example

[nagilson]

Great work updating things in response to my feedback above; if there's anything I can clarify then please let me know, happy to chat! 😊

[marcpopMSFT]

Note: Long term effort that needs reevaluation for our backlog.