add support for GitHub / Azure Artefacts

[JoC0de]

• allow fetching updates using registrationsBaseUrl
• getting single NuGet packages using registrationsBaseUrl
• add the new settings to Settings-UI

fixes #598
and fixes #589

[igor84]

If you are using string interpolation here then perhaps a cleaner solution instead of StringBuilder is to just build a List and then use string.Join(" ", list).

[JoC0de]

We would need:

var query = string.Join(' ', packagesToFetch.Skip(i).Take(UpdateSearchBatchSize).Select(package => $"{package.Id}_"));
i += UpdateSearchBatchSize;

As there is no Chunk method in older C# we need the index. Not sure if the Unity implementation of the Skip already has optimization for List. Do you think it would make the code more readable, with the Skip / Take?

Another think is we could use AppendFormat as none .Net Core doesn't has the Append overload that takes a AppendInterpolatedStringHandler.

[igor84]

I just thought to replace StringBuilder with List and ToString with string.join leaving everything else the same.