refactor(BA-502): Add the skeleton interface of vfolder CRUD handlers in storage-proxy (#3434)

[MintCat98]

resolves #3434 (BA-502)

Add skeletion interface of storage-proxy handlers for refactoring.

Files were added in ./src/ai/backend/storage/api/vfolder

init.py
manager_handler.py
manager_service.py
types.py

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[HyeockJinKim]

Please check the reason for the CI failure as well.

[MintCat98]

Please check the reason for the CI failure as well.

I made some changes before committing, but I found that I only ran pants check :: and forgot to run pants test. I'll fix the test errors along with other modifications and ping you again.