Add a subcommand to dotnetsuggest for retrieving a suggestion command from Nuget or equivalent

[MarkMichaelis]

Add an install command to dotnetsuggest that:
Leveraging dotnet global tools downloads a tool that has a command for suggestions. Once install, register the global tools command so that it is associated with a command to which we are adding suggestions.

Proposed command line (initial):
dotnetsuggest install -tool <toolPackageName> -commandPath <commandPath> -suggestionCommand <suggestionCommand>
toolPackageName The nuget package name that provides command suggestions
commandPath The path to the command for which we are registering completions.
suggestionCommand The command to execute in order to return suggestions.

Future:

• Specify the suggestionCommand in the tool package nuspec file so that it isn't required to specify it on the command line.
• Support a static registration file rather than just a global tool for provide suggestion completion
• Update global tools to register suggestion completion automatically when the global tool is installed either using the global tool itself or a dependent Nuget package that provides suggestion completion.

Notes

• By supporting a global tool for suggestion completion immediately we can use System.CommandLine to build the tool immediately - no need to worry about serialization.