refactor(backend): SystemWebhookで送信されるペイロードの型を追加

[samunohito]

What

タイトルの通りです。

Why

fix #14979

Additional info (optional)

• ローカルでunittestとe2eをそれぞれ実行してエラーが無い事を確認
• ローカルで実際にwebhookを送信してエラーにならないことを確認

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md ※動作に影響ないので
• (If possible) Add tests

[github-actions]

このPRによるapi.jsonの差分
差分はありません。
Get diff files from Workflow Page

[codecov]

Codecov Report

Attention: Patch coverage is 79.36508% with 13 lines in your changes missing coverage. Please review.

Project coverage is 39.98%. Comparing base (eef0c89) to head (f10a781).
Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
packages/backend/src/core/WebhookTestService.ts	61.11%	7 Missing ⚠️
...backend/src/core/AbuseReportNotificationService.ts	0.00%	3 Missing ⚠️
packages/backend/src/core/QueueService.ts	57.14%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #14980      +/-   ##
===========================================
- Coverage    41.67%   39.98%   -1.70%     
===========================================
  Files         1565     1561       -4     
  Lines       203008   197290    -5718     
  Branches      3678     3608      -70     
===========================================
- Hits         84596    78878    -5718     
  Misses      117840   117840              
  Partials       572      572              

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[kakkokari-gtyih]

(Off-topic) misskey-jsにWebhookの型を露出させるようにしたらJavaScript/TypeScriptで連携アプリをつくる人に恩恵がありそう