refactor(BA-1409): Apply provisioner pattern to agent

[fregataa]

resolves #4468 (BA-1409)

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]

It seems a bit unnatural to include the kernel_id as a result value.
If another stage needs the kernel_id, it might appear as though there's a dependency on scratch provision stage, and the dependency relationship wouldn't be easily visible.

[fregataa]

Restart (or recover) stages should be implemented separately