Added the class UniqueGift describing a gift that was upgraded to a unique one.

Author: mircoianese

library/src/main/java/com/pengrad/telegrambot/model/gift/unique/UniqueGift.kt

@@ -0,0 +1,11 @@
+package com.pengrad.telegrambot.model.gift.unique
+
+@Suppress("unused")
+data class UniqueGift(
+    @get:JvmName("baseName") val baseName: String,
+    @get:JvmName("name") val name: String,
+    @get:JvmName("number") val number: Int,
+    @get:JvmName("model") val model: UniqueGiftModel,
+    @get:JvmName("symbol") val symbol: UniqueGiftSymbol,
+    @get:JvmName("backdrop") val backdrop: UniqueGiftBackdrop
+)