Added support to get exported file paths

[filzrev]

I want to get exported files that are created by Exporters (e.g. MarkdownExporter's output)

Related logics are implemented by ExporterBase.cs. But it's defined as internal. So it can't be used from external code.

https://github.com/dotnet/BenchmarkDotNet/blob/master/src/BenchmarkDotNet/Exporters/ExporterBase.cs#L51-L55

Is it possible to add supporting feature that get ExportedFiles from benchmark results? For example.

1. Add ExportedFiles information to benchmark results summary.

If it can add ExportedFiles property to existing benchmark summary reports. If it's possible It can be used in a simpler way.

But Exported files are created by following lines. https://github.com/dotnet/BenchmarkDotNet/blob/master/src/BenchmarkDotNet/Running/BenchmarkRunnerClean.cs#L292-L297

So might not possible to modify Summary object on above lines. (Because summary is designed as immutable)

2. Change ExporterBase::GetArtifactFullName API to public.

If GetArtifactFullName API can be changed to public It can get exported files with following code.

var exportedFiles = config.GetExporters().OfType<ExporterBase>().Select(x=> x.GetArtifactFullName(summary))

[!NOTE] Some exporter ( e,g, RPlotExporter) seems that don't inherited from ExporterBase. (that output multiple files?) So it might be better to adding GetArtifactFullNames to IExporter interface.