Add UnsafeSetFormatMetadata in ImageFrameMetadata

Signed-off-by: 舰队的偶像-岛风酱! <frg2089@outlook.com>

Author: frg2089

src/ImageSharp/Metadata/ImageFrameMetadata.cs

@@ -99,6 +99,11 @@ public TFormatFrameMetadata GetFormatMetadata<TFormatMetadata, TFormatFrameMetad
         return newMeta;
     }
 
+    internal void UnsafeSetFormatMetadata(
+        IImageFormat key,
+        IDeepCloneable value)
+        => this.formatMetadata[key] = value;
+
     /// <summary>
     /// Gets the metadata value associated with the specified key.
     /// </summary>