Support cancelling MultiplayerService session joining functions

Is your feature request related to a problem? Please describe.
It takes several seconds to join a session through MultiplayerService. I'd like to let users cancel a connection attempt during the connection process.

Describe the solution you'd like
I'd like to pass cancellation tokens to functions like MultiplayerService.Instance.CreateOrJoinSessionAsync.

Describe alternatives you've considered
I've tried preventing the NetworkManager from connecting to a session if my own connection function wrapping MultiplayerService.Instance.CreateOrJoinSessionAsync is cancelled, but I cannot find a clean way to do this. I have also considered modifying the Unity.Services.Multiplayer package myself, but I would rather not do this.

未指定任何檔案或測試。首先追蹤 MultiplayerService.Instance.CreateOrJoinSessionAsync 和 NetworkManager 的連線流程，以了解需要在哪些地方傳遞取消權杖。完成標準是：使用者可以透過 MultiplayerService 的函式取消進行中的工作階段加入，而不必自行修改套件。