chore: Impl RBAC APIs for scaling group and agent

[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]

[!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. Learn more

• #2873
• #2872 👈
• #2871
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

Join @fregataa and the rest of your teammates on Graphite