[serve] refactor router

[zcin]

Why are these changes needed?

depends on #48232

Pull out some router init parameters and router init logic into create_router.

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[GeneDer]

oh cool, I was about to point out using lambda function for resolve_request_args_func and that it's not awaitable, but you already fixed it XP. Alternitavely we can just use resolve_request_args as the default for resolve_request_args_func then we don't need another definition elsewhere