Support attribute of array of FileInfo or DirectoryInfo

[WojciechNagorski]

In this PR I would like to support validation of:

new Argument<FileInfo[]>
                {
                    Arity = ArgumentArity.ZeroOrMore
                }.ExistingOnly()

and

new Argument<DirectoryInfo[]>
                    {
                        Arity = ArgumentArity.ZeroOrMore
                    }.ExistingOnly()

I validate until I find the first error. If you want, I can always validate all values.

Thanks to this PR you can pass:
--paths file1.txt file2.txt

I want to use this feature in BenchmarkDotNet in dotnet/BenchmarkDotNet#1016. Currently It has:
https://github.com/dotnet/BenchmarkDotNet/blob/adde64cbbcde01938d6b2772066852c4f6c0e88d/src/BenchmarkDotNet/ConsoleArguments/CommandLineOptions.cs#L80-L81