Add GetStream to IWorkingBeatmap

This is necessary to obtain the filesize of the audio and background files.

osu.Game.Tests/Beatmaps/Formats/LegacyBeatmapEncoderTest.cs

@@ -168,6 +168,8 @@ namespace osu.Game.Tests.Beatmaps.Formats
             protected override Texture GetBackground() => throw new NotImplementedException();
 
             protected override Track GetBeatmapTrack() => throw new NotImplementedException();
+
+            public override Stream GetStream(string storagePath) => throw new NotImplementedException();
         }
     }
 }