Add Single and Dual Relation Property type

Author: KoditkarVedant

Src/Notion.Client/Models/Database/Properties/RelationProperty.cs

@@ -0,0 +1,21 @@
+using Newtonsoft.Json;
+
+namespace Notion.Client
+{
+    public class DualPropertyRelation : RelationData
+    {
+        public override RelationType Type => RelationType.Dual;
+
+        [JsonProperty("dual_property")]
+        public Data DualProperty { get; set; }
+
+        public class Data
+        {
+            [JsonProperty("synced_property_name")]
+            public string SyncedPropertyName { get; set; }
+
+            [JsonProperty("synced_property_id")]
+            public string SyncedPropertyId { get; set; }
+        }
+    }
+}

Src/Notion.Client/Models/Database/Properties/RelationProperty/DualPropertyRelation.cs

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+using JsonSubTypes;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
+
+namespace Notion.Client
+{
+    [JsonConverter(typeof(JsonSubtypes), "type")]
+    [JsonSubtypes.KnownSubType(typeof(SinglePropertyRelation), RelationType.Single)]
+    [JsonSubtypes.KnownSubType(typeof(DualPropertyRelation), RelationType.Dual)]
+    public abstract class RelationData
+    {
+        [JsonProperty("database_id")]
+        public string DatabaseId { get; set; }
+
+        [JsonProperty("type")]
+        [JsonConverter(typeof(StringEnumConverter))]
+        public virtual RelationType Type { get; set; }
+    }
+}

Src/Notion.Client/Models/Database/Properties/RelationProperty/RelationData.cs

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+using System.Collections.Generic;
+using System.Runtime.Serialization;
+using Newtonsoft.Json;
+
+namespace Notion.Client
+{
+    public class RelationProperty : Property
+    {
+        public override PropertyType Type => PropertyType.Relation;
+
+        [JsonProperty("relation")]
+        public RelationData Relation { get; set; }
+    }
+}

Src/Notion.Client/Models/Database/Properties/RelationProperty/RelationProperty.cs

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+using System.Runtime.Serialization;
+
+namespace Notion.Client
+{
+    public enum RelationType
+    {
+        [EnumMember(Value = "single_property")]
+        Single,
+
+        [EnumMember(Value = "dual_property")]
+        Dual
+    }
+}

Src/Notion.Client/Models/Database/Properties/RelationProperty/RelationType.cs

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+using System.Collections.Generic;
+using Newtonsoft.Json;
+
+namespace Notion.Client
+{
+    public class SinglePropertyRelation : RelationData
+    {
+        public override RelationType Type => RelationType.Single;
+
+        [JsonProperty("single_property")]
+        public Dictionary<string, object> SingleProperty { get; set; }
+    }
+}