Download file using WebClient

Right now, the only way to download a file from Slack is to do something like this:

import requests
response = requests.get(url_private, headers={"Authorization": f"Bearer {TOKEN}"}, timeout=30)

However, this does not respects the parameters passed to WebClient. We should be able to do something like:

client = WebClient()
response = client.get(url_private)

Or something along those lines.

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 by reading the slack_sdk.web.WebClient sync and async implementations and tracing how they apply client parameters to requests. Check the existing Web API client tests if present, then define completion as allowing file downloads through WebClient while preserving its configured parameters and supporting both client variants.