Add eager_tasks parameter to asyncio Runner and run()

Feature or enhancement

Proposal:

We have an agreement that eager tasks should become defaults in asyncio eventually.

This issue implements the first step: it adds opt-in eager_tasks argument to run() function and Runner constructor.
Later we'll deprecate False value and enforce users to switch to eager tasks in their code.

See the linked discussion for details.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/make-asyncio-eager-task-factory-default/75164/10

Linked PRs

• gh-128293

まずリンク先のDiscourseディスカッションを読み、次にCPythonのasyncio実装内でasyncio.run()とRunnerコンストラクターを見つけてください。オプトイン引数eager_tasksについて合意された動作を確認し、両方のエントリーポイントが仕様どおりにそれを受け付けて適用することを検証してください。