Add support for slash commands option errors (#162)

* Add support for slash commands option errors

* nested errors has no definite structure

rest/src/main/kotlin/json/response/DiscordErrorResponse.kt

@@ -1,33 +1,72 @@
 package dev.kord.rest.json.response
 
+import dev.kord.common.entity.optional.Optional
+import dev.kord.common.entity.optional.optional
 import dev.kord.rest.json.JsonErrorCode
+import kotlinx.serialization.KSerializer
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.builtins.ListSerializer
+import kotlinx.serialization.builtins.MapSerializer
+import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonDecoder
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.jsonObject
 
 /**
  * Represents a [Discord error response](https://discord.com/developers/docs/topics/opcodes-and-status-codes#json).
  */
 @Serializable
 class DiscordErrorResponse(
-        val code: JsonErrorCode = JsonErrorCode.Unknown,
-        val errors: Map<String, DiscordFieldError> = emptyMap(),
-        val message: String = "",
+    val code: JsonErrorCode = JsonErrorCode.Unknown,
+    val errors: Map<String, DiscordFieldError> = emptyMap(),
+    val message: String = "",
 )
 
 /**
  * An error for a specific field.
  */
-@Serializable
+@Serializable(DiscordFieldError.Serializer::class)
 class DiscordFieldError(
-        @SerialName("_errors")
-        val errors: List<DiscordErrorDetail>,
-)
+    @SerialName("_errors")
+    val errors: List<DiscordErrorDetail> = emptyList(),
+    val nestedErrors: Optional<JsonObject> = Optional.Missing()
+) {
+    internal object Serializer : KSerializer<DiscordFieldError> {
+
+        override val descriptor: SerialDescriptor
+            get() = JsonObject.serializer().descriptor
+
+
+        override fun deserialize(decoder: Decoder): DiscordFieldError {
+            decoder as JsonDecoder
+            val json = decoder.decodeJsonElement().jsonObject
+            // Direct Error message
+            if (json.containsKey("_errors"))
+                return DiscordFieldError(
+                    Json.decodeFromJsonElement(ListSerializer(DiscordErrorDetail.serializer()), json["_errors"]!!)
+                )
+
+            // Nested fields have no definite structure.
+            return DiscordFieldError(nestedErrors = json.optional())
+        }
+
+        override fun serialize(encoder: Encoder, value: DiscordFieldError) {
+            TODO("Not yet implemented")
+        }
+
+    }
+}
 
 /**
  * The detailed code and message for a [DiscordFieldError].
  */
 @Serializable
 class DiscordErrorDetail(
-        val code: String,
-        val message: String,
+    val code: String,
+    val message: String,
 )