feat: define Instance model

[laminne]

close #831

What does this PR do?

• Instanceモデルの定義をしました
• (クラスの)インスタンス生成チェックにスナップショットテストを導入しました

Additional information

[codecov]

Codecov Report

Attention: Patch coverage is 67.45562% with 55 lines in your changes missing coverage. Please review.

Project coverage is 49.33%. Comparing base (e8aa6ce) to head (ad408e1).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/federation/model/instance.ts	58.33%	55 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #837      +/-   ##
==========================================
+ Coverage   49.13%   49.33%   +0.19%     
==========================================
  Files         165      167       +2     
  Lines       15435    15604     +169     
  Branches     1211     1221      +10     
==========================================
+ Hits         7584     7698     +114     
- Misses       7851     7906      +55     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.