fix: custom emoji support

Author: mzahor

Src/Notion.Client/Models/CustomEmoji.cs

@@ -0,0 +1,16 @@
+using Newtonsoft.Json;
+
+namespace Notion.Client
+{
+    public class CustomEmoji
+    {
+        [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/CustomEmojiObject.cs

@@ -0,0 +1,13 @@
+using Newtonsoft.Json;
+
+namespace Notion.Client
+{
+    public class CustomEmojiObject : IPageIcon
+    {
+        [JsonProperty("custom_emoji")]
+        public CustomEmoji CustomEmoji { get; set; }
+
+        [JsonProperty("type")]
+        public string Type { get; set; }
+    }
+}

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

@@ -5,6 +5,7 @@ namespace Notion.Client
 {
     [JsonConverter(typeof(JsonSubtypes), "type")]
     [JsonSubtypes.KnownSubTypeAttribute(typeof(EmojiObject), "emoji")]
+    [JsonSubtypes.KnownSubTypeAttribute(typeof(CustomEmojiObject), "custom_emoji")]
     [JsonSubtypes.KnownSubTypeAttribute(typeof(FileObject), "file")]
     [JsonSubtypes.KnownSubTypeAttribute(typeof(FileObject), "external")]
     public interface IPageIcon