feat: Add UniqueGiftModel type

tgbotapi.core/src/commonMain/kotlin/dev/inmo/tgbotapi/types/UniqueGiftModel.kt

@@ -0,0 +1,22 @@
+package dev.inmo.tgbotapi.types
+
+import dev.inmo.tgbotapi.types.files.Sticker
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * This object describes the model of a unique gift.
+ *
+ * @param name Name of the model
+ * @param sticker The sticker that represents the unique gift
+ * @param rarityPerMille The number of unique gifts that receive this model for every 1000 gifts upgraded
+ */
+@Serializable
+data class UniqueGiftModel(
+    @SerialName(nameField)
+    val name: String,
+    @SerialName(stickerField)
+    val sticker: Sticker,
+    @SerialName(rarityPerMilleField)
+    val rarityPerMille: Int
+)