feat: define Notification model

[laminne]

close #874

What does this PR do?

• Notificationモデルの定義
  • Announceモデルはv0.1では取り敢えずやらないことにします

Additional information

[codecov]

Codecov Report

Attention: Patch coverage is 73.13433% with 18 lines in your changes missing coverage. Please review.

Project coverage is 49.71%. Comparing base (26dd98c) to head (cec7405).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/notification/model/notification.ts	65.38%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #876      +/-   ##
==========================================
+ Coverage   49.47%   49.71%   +0.24%     
==========================================
  Files         177      179       +2     
  Lines       15981    16141     +160     
  Branches     1251     1268      +17     
==========================================
+ Hits         7906     8025     +119     
- Misses       8075     8116      +41     

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