start migration onto ktor-based serialization of data

features/content/common/src/commonMain/kotlin/dev/inmo/postssystem/features/content/common/Content.kt

@@ -1,7 +1,6 @@
 package dev.inmo.postssystem.features.content.common
 
-import dev.inmo.micro_utils.common.FileName
-import dev.inmo.micro_utils.common.MPPFile
+import dev.inmo.micro_utils.common.*
 import dev.inmo.micro_utils.mime_types.MimeType
 import dev.inmo.postssystem.features.common.common.SimpleInputProvider
 import kotlinx.serialization.PolymorphicSerializer
@@ -30,6 +29,18 @@ data class BinaryContent(
 ) : Content
 
 val ContentSerializer = PolymorphicSerializer(Content::class)
+@Serializable
+data class ContentWrapper(
+    val content: Content
+)
+@Serializable
+data class ContentsWrapper(
+    val content: List<Content>
+)
+@Serializable
+data class ContentsEithersWrapper(
+    val content: List<Either<ContentId, Content>>
+)
 
 /**
  * Content which is already registered in database. Using its [id] you can retrieve all known