Add author_subname to attachment models

[seratch]

Summary

This pull request adds author_subname to attachment model classes. See slackapi/node-slack-sdk#1621 for the context.

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

Merging #1366 (89ac6e4) into main (7eec685) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 89ac6e4 differs from pull request most recent head ffa4057. Consider uploading reports for the commit ffa4057 to get more accurate results

@@           Coverage Diff           @@
##             main    #1366   +/-   ##
=======================================
  Coverage   85.58%   85.59%           
=======================================
  Files         111      111           
  Lines       11559    11560    +1     
=======================================
+ Hits         9893     9895    +2     
+ Misses       1666     1665    -1     

Impacted Files	Coverage Δ
slack_sdk/models/attachments/__init__.py	99.41% <100.00%> (+<0.01%)	⬆️

... and 2 files with indirect coverage changes

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