feat: support DeepL's model (close #1126)

relay/constant/common.go

@@ -1,3 +1,5 @@
 package constant
 
 var StopFinishReason = "stop"
+var StreamObject = "chat.completion.chunk"
+var NonStreamObject = "chat.completion"

relay/constant/finishreason/define.go

@@ -0,0 +1,5 @@
+package finishreason
+
+const (
+	Stop = "stop"
+)

relay/constant/role/define.go

@@ -0,0 +1,5 @@
+package role
+
+const (
+	Assistant = "assistant"
+)