Make ChatCompletionRequest a data class (#399)

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Author: yunmanger1

openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatCompletionRequest.kt

@@ -14,7 +14,7 @@ import kotlinx.serialization.json.JsonObjectBuilder
  * Creates a completion for the chat message.
  */
 @Serializable
-public class ChatCompletionRequest(
+public data class ChatCompletionRequest(
     /**
      * ID of the model to use.
      */