[Malleability] Update Backend to work with BackData

[UlyanaAndrukhiv]

Context

The Backend currently relies on BackData using fixed types (flow.Identifier and flow.Entity). However, there is a planned update outlined in flow-go#7070 to refactor BackData to use generic arguments instead. This change is expected to improve flexibility but will break the current Backend implementation if it is not updated accordingly. The Backend assumes a fixed interface, so when the generic implementation is introduced, discrepancies in type expectations will cause integration issues.

Proposed Solution

Once the changes in flow-go#7070 are finalized the Backend must be refactored to accommodate the new generic parameters introduced in BackData. This will involve identifying all Backend components that interact with BackData and modifying them to handle generic types. The solution should ensure that the Backend correctly interprets and processes the generic data without compromising functionality.

Defenition of done

• The Backend is refactored to integrate seamlessly with the updated generic BackData implementation.
• All code using Backend is updated to work with the new generic design.
• Documentation is updated to reflect the changes in the Backend interaction with BackData.
• Tests are updated or added to ensure that the new design functions correctly without breaking existing behavior.

Related issues

#6703
#7070