feat: implement network agent plugin architecture

[kyujin-cho]

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

[kyujin-cho]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• feat: restrict maximum number of networks available per project #2857
• feat: create network management CLI commands #2800
• feat: update agent to utilize network plugin  #2100
• feat: implement network agent plugin architecture #2099 👈
• feat: update manager to utilize network plugin #2098
• feat: implement network manager plugin architecture #2097
• feat: add support for creating and managing networks #2726
• main

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