feat: add new `conversations.externalInvitePermissions.set` API

[WilliamBergamin]

Summary

This is a duplicate of #1516 and adds support for https://api.slack.com/methods/conversations.externalInvitePermissions.set

I did not get a chance to test out the integration tests locally, before noticing this was a duplicate

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

• [x] 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 [ ])

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

[filmaj]

Yours has integration tests! Better than my PR. Let's use this PR instead.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.90%. Comparing base (e64677b) to head (8548f56).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1517      +/-   ##
==========================================
+ Coverage   84.89%   84.90%   +0.01%     
==========================================
  Files         113      113              
  Lines       12480    12489       +9     
==========================================
+ Hits        10595    10604       +9     
  Misses       1885     1885              

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