fix: optional dependency install

[WilliamBergamin]

Summary

This PR aims to fix #1455 by exposing the dependencies found in requirements/optional.txt

I've deployed these changes to https://test.pypi.org/project/slack-sdk/3.26.3a0/

You can test them out by executing pip3 install -i https://test.pypi.org/simple/ 'slack-sdk[optional]==3.26.3a0'
NOTE: the install will fail since not all of the dependencies in requirements/optional.txt are in test.pypi.org

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.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dc96f5f) 85.34% compared to head (47a0f0d) 85.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1456      +/-   ##
==========================================
- Coverage   85.34%   85.34%   -0.01%     
==========================================
  Files         111      111              
  Lines       12273    12273              
==========================================
- Hits        10475    10474       -1     
- Misses       1798     1799       +1     

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[seratch]

Thank you!