Allow multiple FileDataSourceAttribute to be used

Simply removing this will allow plugins to support multiple different file extensions. WPA already has the support for it as WPA always processes `FileDataSourceAttribute` in an `IEnumerable`.
Even in this repo we have tests for multiple extensions (`TryGetFileExtensions_FileDataSources_ReturnsExtensions`)

Author: xj-ms

src/Microsoft.Performance.SDK/Processing/FileDataSourceAttribute.cs

@@ -12,7 +12,7 @@ namespace Microsoft.Performance.SDK.Processing
     ///     files of a specific type. This is used to instruct callers to route files
     ///     with a given extension to the decorated <see cref="IProcessingSource"/> for processing.
     /// </summary>
-    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
+    [AttributeUsage(AttributeTargets.Class)]
     public sealed class FileDataSourceAttribute
         : DataSourceAttribute,
           IEquatable<FileDataSourceAttribute>