feat: progress report for image pulling

[fregataa]

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

[fregataa]

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• feat: progress report for image pulling #2728 👈
• chore: Add sub-progress reporter #2727
• feat: Apply check-and-pull mechanism to session creation lifecycle #2721
• feat: Add check-and-pull RPC function  #2692
• feat: Add new sessions & kernels status #2647 : 1 other dependent PR (#2883 )
• refactor: image configuration when create kernels #2646
• feat: Add session status checker API #2312
• refactor: revamp update mechanism of session & kernel status #2311
• feat: Add interface that update session and kernel status #2581
• main

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