add custom MediaGroupMemberInputMedia serializer

src/main/kotlin/com/github/insanusmokrassar/TelegramBotAPI/types/InputMedia/MediaGroupMemberInputMedia.kt

@@ -1,7 +1,9 @@
 package com.github.insanusmokrassar.TelegramBotAPI.types.InputMedia
 
+import kotlinx.serialization.Serializable
 import kotlinx.serialization.StringFormat
 
+@Serializable(MediaGroupMemberInputMediaSerializer::class)
 interface MediaGroupMemberInputMedia: InputMedia {
     fun serialize(format: StringFormat): String
     val arguments: Map<String, Any?>

src/main/kotlin/com/github/insanusmokrassar/TelegramBotAPI/types/InputMedia/MediaGroupMemberInputMediaSerializer.kt

@@ -0,0 +1,13 @@
+package com.github.insanusmokrassar.TelegramBotAPI.types.InputMedia
+
+import kotlinx.serialization.*
+
+@Serializer(MediaGroupMemberInputMedia::class)
+object MediaGroupMemberInputMediaSerializer : KSerializer<MediaGroupMemberInputMedia> {
+    override fun serialize(encoder: Encoder, obj: MediaGroupMemberInputMedia) {
+        when (obj) {
+            is InputMediaPhoto -> InputMediaPhoto.serializer().serialize(encoder, obj)
+            is InputMediaVideo -> InputMediaVideo.serializer().serialize(encoder, obj)
+        }
+    }
+}