Add block as parent for page and database

Src/Notion.Client/Models/Database/IDatabaseParent.cs

@@ -6,6 +6,7 @@ namespace Notion.Client
     [JsonConverter(typeof(JsonSubtypes), "type")]
     [JsonSubtypes.KnownSubType(typeof(PageParent), ParentType.PageId)]
     [JsonSubtypes.KnownSubType(typeof(WorkspaceParent), ParentType.Workspace)]
+    [JsonSubtypes.KnownSubType(typeof(BlockParent), ParentType.BlockId)]
     public interface IDatabaseParent : IParent
     {
     }

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

@@ -7,6 +7,7 @@ namespace Notion.Client
     [JsonSubtypes.KnownSubType(typeof(DatabaseParent), ParentType.DatabaseId)]
     [JsonSubtypes.KnownSubType(typeof(PageParent), ParentType.PageId)]
     [JsonSubtypes.KnownSubType(typeof(WorkspaceParent), ParentType.Workspace)]
+    [JsonSubtypes.KnownSubType(typeof(BlockParent), ParentType.BlockId)]
     public interface IPageParent : IParent
     {
     }

Src/Notion.Client/Models/Parents/BlockParent.cs

@@ -0,0 +1,18 @@
+using Newtonsoft.Json;
+
+namespace Notion.Client
+{
+    public class BlockParent : IPageParent, IDatabaseParent
+    {
+        /// <summary>
+        /// Always has a value "block_id"
+        /// </summary>
+        public ParentType Type { get; set; }
+
+        /// <summary>
+        /// The ID of the block that the element belongs to.
+        /// </summary>
+        [JsonProperty("block_id")]
+        public string BlockId { get; set; }
+    }
+}

Src/Notion.Client/Models/Parents/ParentType.cs

@@ -14,6 +14,9 @@ public enum ParentType
         PageId,
 
         [EnumMember(Value = "workspace")]
-        Workspace
+        Workspace,
+
+        [EnumMember(Value = "block_id")]
+        BlockId,
     }
 }