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

• [x] Read the contribution guide
• [x] 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[bot]]

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[bot]]

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 16 commits behind head on develop.

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

Additional details and impacted files

```diff @@ 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

[syuilo]

🙏