All fluent APIs defined by Option<T> and Argument<T> should return generic value

[adamsitnik]

While working on unblocking SDK with latest S.CL update in dotnet/sdk#29012 I've realized that in #1951 we have moved argument and option extension methods to the types themselves, but unfortunately, we have also made these methods non-generic. It does not allow to re-use the returned value when we need a generic value.

Example:

public static readonly Argument<string> CmdPackageArgument = new Argument<string>(LocalizableStrings.CmdPackage)
{
    Description = LocalizableStrings.CmdPackageDescription
}.AddCompletions((context) => QueryNuGet(context.WordToComplete).Select(match => new CompletionItem(match)));

// currently `AddCompletions` returns an `Argument` rather than `Argument<string>`

 Cannot implicitly convert type 'System.CommandLine.Argument' to 'System.CommandLine.Argument<string>'. An explicit
 conversion exists

cc @jozkee @jonsequitur