fix: Add explicit legacy format converting for config types

[jopemachine]

resolves #NNN (BA-MMM)

WIP

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]

Can you guarantee that this is the same as the value you received as input?
I'm not sure if it's easy to change it due to plugins.