ToData now supports multiple formats.

Introduce `ProgrartC` to build a single image to work with other build tools.
Added a `CombinedStorageProvider` to allow aggregate storage providers.

Progrart.Core/Storage/CombinedStorageProvider.cs

@@ -0,0 +1,26 @@
+namespace Progrart.Core.Storage
+{
+    public class CombinedStorageProvider : IStorageProvider
+	{
+		public List<IStorageProvider> providers = new List<IStorageProvider>();
+		public async Task<Stream?> TryOpenRead(string path)
+		{
+			foreach (var item in providers)
+			{
+				var v = await item.TryOpenRead(path);
+				if (v != null) return v;
+			}
+			return null;
+		}
+
+		public async Task<Stream?> TryOpenWrite(string path)
+		{
+			foreach (var item in providers)
+			{
+				var v = await item.TryOpenWrite(path);
+				if (v != null) return v;
+			}
+			return null;
+		}
+	}
+}