Added support for "custom_emoji" page icons

Src/Notion.Client/Models/CustomEmojiObject.cs

@@ -0,0 +1,25 @@
+using Newtonsoft.Json;
+
+namespace Notion.Client
+{
+    public class CustomEmojiObject : IPageIcon
+    {
+        [JsonProperty("type")]
+        public virtual string Type { get; set; }
+
+        [JsonProperty("custom_emoji")]
+        public Info Emoji { get; set; }
+
+        public class Info
+        {
+            [JsonProperty("id")]
+            public string Id { get; set; }
+
+            [JsonProperty("name")]
+            public string Name { get; set; }
+
+            [JsonProperty("url")]
+            public string Url { get; set; }
+        }
+    }
+}

Src/Notion.Client/Models/Page/IPageIcon.cs

@@ -7,6 +7,7 @@ namespace Notion.Client
     [JsonSubtypes.KnownSubTypeAttribute(typeof(EmojiObject), "emoji")]
     [JsonSubtypes.KnownSubTypeAttribute(typeof(FileObject), "file")]
     [JsonSubtypes.KnownSubTypeAttribute(typeof(FileObject), "external")]
+    [JsonSubtypes.KnownSubTypeAttribute(typeof(CustomEmojiObject), "custom_emoji")]    
     public interface IPageIcon
     {
         [JsonProperty("type")]