add: demo 进程插件配置（load_mode=external, workers.demo）

workers/demo/hello.toml

@@ -0,0 +1,9 @@
+[GET]
+summary = "进程插件示例：问候"
+params = ["name: str"]
+auth_required = false
+
+[GET.response_model]
+greeting = { type = "str", required = true }
+worker_id = { type = "str", required = false, default = "" }
+timestamp = { type = "float", required = true }