Improve structure of `admin.apps.config.set` HTTP API arguments

I improved the documentation and helped implement the API methods for admin.apps.config.* recently in the node-slack-sdk. As part of this work, it became clear that the arguments for the admin.apps.config.set method were vaguely documented and unclear.

I did some discovery and was able to more specifically type them in the node-slack-sdk. See this section of the PR in node-slack-sdk.

Perhaps we could do something similar in the Python SDK.

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

• slack_sdk.web.WebClient (sync/async) (Web API client)