fix(chat): add systemFingerprint to ChatCompletionChunk (#303)

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

@@ -1,6 +1,5 @@
 package com.aallam.openai.api.chat
 
-import com.aallam.openai.api.BetaOpenAI
 import com.aallam.openai.api.core.Usage
 import com.aallam.openai.api.model.ModelId
 import kotlinx.serialization.SerialName
@@ -42,4 +41,11 @@ public data class ChatCompletionChunk(
      */
     @SerialName("usage")
     public val usage: Usage? = null,
+
+    /**
+     * This fingerprint represents the backend configuration that the model runs with. Can be used in conjunction with
+     * the `seed` request parameter to understand when backend changes have been made that might impact determinism.
+     */
+    @SerialName("system_fingerprint")
+    public val systemFingerprint: String? = null,
 )