[Malleability] Update BackData to use generic arguments instead of flow.Identifier and flow.Entity

[UlyanaAndrukhiv]

Context

The current implementation of the BackData interface uses flow.Identifier and flow.Entity. The primary concern here is the unnecessary dependency on flow.Entity. The goal is to refactor BackData to eliminate this dependency by using generic arguments instead.

Proposed Solution

Refactor the BackData interface to use generic type parameters instead of depending on flow.Identifier and flow.Entity. This change will eliminate the misuse of flow.Entity.

Defenition of done

• The BackData interface is refactored to use generic type parameters instead of flow.Identifier and flow.Entity.
• All code using BackData is updated to work with the new generic design.
• Tests are updated or added to ensure that the new design functions correctly without breaking existing behavior.

Related issues

#6703