[feature request] add loop._run_once and loop._set_is_runnning api to easy the process of OS gui event loop integrate

• uvloop version:
  latest

• Python version:
  3.11.2

• Platform:
  All, and testing on Linux

• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?:
  n.a

• Does uvloop behave differently from vanilla asyncio? How?:
  no

• Purpose:
  Add a public API to

1. setting running state
2. allow run_once of event loop

• Background:

I want to pull the event from backend_fd from another process and process them on os GUI event loop which like electron, but uvloop has no public API to run_once and keep _is_running

• Refs:

1. https://discuss.python.org/t/connecting-asyncio-and-tkinter-event-loops/14722/9
2. https://github.com/congzhangzh/webview_python/issues/1

首先检查所请求的入口点 loop._run_once 和 loop._set_is_runnning，以及 issue 中描述的 backend_fd 集成。阅读链接的 asyncio/Tkinter 讨论，以了解所提议的事件循环交互。完成条件是确定一个支持设置运行状态并为 OS GUI 集成运行一次迭代的公共 API。