feat: Add custom_message to AutoModActionMetadata and fix TypeError on AutoModRule

[NeloBlivion]

Summary

• Adds AutoModActionMetadata.custom_message as per [automod] Add custom_message metadata field discord/discord-api-docs#5955
• Adds __dict__ to AutoModRule.__slots__ to close on_auto_moderation_rule_X events TypeError #1976

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.

[codecov]

Codecov Report

Merging #2029 (3165db0) into master (0407f28) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2029      +/-   ##
==========================================
- Coverage   33.30%   33.29%   -0.01%     
==========================================
  Files          97       97              
  Lines       18935    18940       +5     
==========================================
  Hits         6307     6307              
- Misses      12628    12633       +5     

Flag	Coverage Δ
macos-latest-3.10	33.28% <0.00%> (-0.01%)	⬇️
macos-latest-3.11	33.28% <0.00%> (-0.01%)	⬇️
macos-latest-3.8	33.29% <0.00%> (-0.01%)	⬇️
macos-latest-3.9	33.29% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.10	33.28% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.11	33.28% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.8	33.29% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.9	33.29% <0.00%> (-0.01%)	⬇️
windows-latest-3.10	33.28% <0.00%> (-0.01%)	⬇️
windows-latest-3.11	33.28% <0.00%> (-0.01%)	⬇️
windows-latest-3.8	33.29% <0.00%> (-0.01%)	⬇️
windows-latest-3.9	33.29% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
discord/automod.py	27.97% <0.00%> (-0.86%)	⬇️

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[pullapprove4]

Please add a changelog entry