Export types for the objects returned from making requests

The export of types of the objects returned when making requests would be incredibly useful. This would significantly enhance the developer experience by:

• preventing incorrect key accesses
• and other invalid operations on the type of an object or value inside the object
• enhancing IDE intellisense
• integration with linters and static type checkers

I guess you could implement this manually, though it would be amazing if the Slack API did this for you.

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

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

Start with slack_sdk.web.WebClient, the only selected client area in the issue, and read the linked contributing guidelines before investigating how Web API responses are represented. Define the scope of exported response types and how they would cover sync and async requests; done should provide usable types that support key checking, IDE completion, linters, and static type checkers.