feat: Add UniqueGiftBackdrop type

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

@@ -0,0 +1,21 @@
+package dev.inmo.tgbotapi.types
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * This object describes the backdrop of a unique gift.
+ *
+ * @param name Name of the backdrop
+ * @param colors Colors of the backdrop
+ * @param rarityPerMille The number of unique gifts that receive this backdrop for every 1000 gifts upgraded
+ */
+@Serializable
+data class UniqueGiftBackdrop(
+    @SerialName(nameField)
+    val name: String,
+    @SerialName(colorsField)
+    val colors: UniqueGiftBackdropColors,
+    @SerialName(rarityPerMilleField)
+    val rarityPerMille: Int
+)