feat: Add RBAC Entity Creator

[fregataa]

resolves #6417 (BA-2839)

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's easy to miss this, so it would be good to apply the matters related to scope and entity as abstractmethod.

[Copilot]

Pull Request Overview

This PR introduces an RBAC (Role-Based Access Control) entity creator that manages the association between scopes and entities in the system. The implementation provides a reusable base class for creating RBAC-related database entries with proper conflict handling.

• Introduces RBACCreator as an abstract base class for RBAC entity creation operations
• Implements automatic scope-entity association creation with conflict resolution
• Adds logging for integrity constraint violations during RBAC row creation

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The IntegrityError exception is caught after .on_conflict_do_nothing() is used, making this catch block unreachable. The on_conflict_do_nothing() clause prevents IntegrityError from being raised for conflicts. Remove the try-except block or remove the on_conflict_do_nothing() clause if you need to handle conflicts explicitly.

Suggested change

	@abstractmethod
	def scope_id(self) -> ScopeId:
	raise NotImplementedError
	@abstractmethod
	def object_id(self) -> ObjectId:
	raise NotImplementedError
	TCreatedEntity = TypeVar("TCreatedEntity")
	await db_session.execute(
	pg_insert(AssociationScopesEntitiesRow)
	.values(creator.fields_to_store())
	.on_conflict_do_nothing()
	)