feat(credentials): Implement KMS encryption process

[vprashrex]

Issue

Closes #416

Summary

• Before: Existing credentials were not encrypted with KMS.
• Now: Added KMS-based encryption and re-encryption process for credentials.
• Implemented integration with KMS for encryption.
• Developed re-encryption logic to update existing credentials.

Checklist

Before submitting a pull request, please ensure that you mark these task.

• Ran fastapi run --reload app/main.py or docker compose up in the repository root and test.
• If you've fixed a bug or added code that is tested and has test cases.

Notes

Please read the documentation for better understanding of how kms will work in kaapi
https://docs.google.com/document/d/1N8KVB6de0sydwtXvgJKNAjGKrZ5Uke8IUoWPDyNWMqc/edit?tab=t.0#heading=h.t2mkgsxvvisn

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• Commit unit tests in branch feat/kms-integration

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[github-actions]

OpenAPI changes   ⚪ No API surface changes

Note

This PR does not modify the API contract.

_{main ↔ 60832991 · generated by oasdiff}

[codecov]

Codecov Report

❌ Patch coverage is 85.03401% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/app/core/security.py	71.79%	11 Missing ⚠️
backend/app/services/credentials/reencrypt.py	76.31%	9 Missing ⚠️
backend/app/core/cloud/storage.py	50.00%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

🎉 This PR is included in version 1.4.0-main.4 🎉

The release is available on GitHub release

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