Please add `app.get_new_client()` to help with token rotation

From a discussion in #1324, the only safe way to call the client from outside a bolt handler when token rotation is on is to create a new client each time (as the handlers do)

Since Bolt is currently handling all of this for us, it'd be very useful to have a method along the lines of

client = await bolt.get_new_client(team_id)

that means callers do not need to deal with the InstallationStore and such themselves.

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Others

Requirements

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先查看讨论 #1324，然后检查入口点 slack_bolt.App 和 slack_bolt.async_app.AsyncApp，以及它们在 token 轮换期间如何使用 InstallationStore。完成的标准是，调用方无需自行处理 store，即可通过所请求的方法获取为某个团队新创建的客户端，并且同步和异步应用都必须支持这一点。