refactor: centralize llm backend configuration

plugins/dify/config.toml

@@ -1,8 +1,6 @@
 [Dify]
 enable = true
-
-api-key = "app-u5EnYq3ill19bm6pWJwGkY4D" # Dify的API Key
-base-url = "http://192.168.2.240/v1" #Dify API接口base url
+backend = "dify_workflow_chat"
 
 commands = ["聊天"]
 command-tip = """
@@ -17,4 +15,4 @@ http-proxy = ""
 
 # 管理员和白名单用户是否免费使用
 admin_ignore = true
-whitelist_ignore = true
+whitelist_ignore = true