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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Review discussion #1324 first, then inspect the slack_bolt.App and slack_bolt.async_app.AsyncApp entry points and how they use InstallationStore during token rotation. Done means callers can obtain a newly created client for a team through the requested methods without handling the store themselves, for both synchronous and asynchronous apps.