Fix #982 Add admin.users.session.* API support

[seratch]

Summary

This pull request fixes #982 - refer to the issue for details.

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)
• /docs-src (Documents, have you run ./docs.sh?)
• /docs-src-v2 (Documents, have you run ./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 python setup.py validate after making the changes.

[codecov]

Codecov Report

Merging #984 (c57abbd) into main (67bac12) will increase coverage by 0.00%.
The diff coverage is 84.12%.

@@           Coverage Diff           @@
##             main     #984   +/-   ##
=======================================
  Coverage   87.24%   87.24%           
=======================================
  Files          94       94           
  Lines        8543     8588   +45     
=======================================
+ Hits         7453     7493   +40     
- Misses       1090     1095    +5     

Impacted Files	Coverage Δ
slack_sdk/web/async_client.py	94.49% <80.95%> (-0.31%)	⬇️
slack_sdk/web/client.py	96.00% <85.71%> (-0.34%)	⬇️
slack_sdk/web/legacy_client.py	95.87% <85.71%> (-0.34%)	⬇️
slack_sdk/socket_mode/builtin/internals.py	73.16% <0.00%> (+0.43%)	⬆️
slack_sdk/socket_mode/client.py	76.99% <0.00%> (+2.65%)	⬆️

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