[router] Add /list_workers endpoint to router

[zhuzilin]

Motivation

In some use case, we may need to send a certain type of request to all workers, e.g. in RL training, we may send /flush_cache or /abort_request to all workers. It will be great to add a /list_workers endpoint to the router, so that we can control workers through the router.

Modifications

Add a /list_workers endpoint, which will return { "urls": worker_list }.

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[ByronHsu]

Thanks!