Support custom package sources from NuGet.config

[menees]

When tools are installed from a custom package source (other than NuGet.org), the tools-outdated command doesn't find them, and it reports the available version as blank/missing. The OutdatedCommand class is hardcoded to use https://api-v2v3search-0.nuget.org/ for its search. It would be nice if it searched the custom package sources declared in nuget.config first.

[menees]

I've stopped using dotnet-tools-outdated. Now, I use a PowerShell script that calls dotnet tool list and dotnet tool list --global. For each tool reported, it calls dotnet package search to find the latest available version. Each command supports --format json, and modern PowerShell parses it fairly easily. This way I don't have to depend on third-party tools, and the built-in dotnet commands already support custom package sources.