feat: add model gemini-2.0-flash-exp

relay/channel/gemini/constant.go

@@ -8,6 +8,7 @@ var ModelList = []string{
 	"gemini-1.5-pro-latest", "gemini-1.5-flash-latest", "gemini-ultra",
 	"gemini-1.5-pro-exp-0827", "gemini-1.5-flash-exp-0827",
 	"gemini-exp-1114", "gemini-exp-1206",
+	"gemini-2.0-flash-exp",
 }
 
 var ChannelName = "google gemini"