[Malleability] Update mapBackData to use generics and implement BackData

[UlyanaAndrukhiv]

Context

Currently implementation of MapBackData is tightly coupled to concrete types (flow.Identifier and flow.Entity) and does not leverage generics. This limits flexibility and type safety, especially as we move toward a more modular design for our BackData implementations. With the upcoming generics overhaul in BackData detailed in flow-go#7070, it is essential to update MapBackData to use generic type parameters. This change will decouple the implementation from fixed types and ensure consistency with the new BackData interface once flow-go#7070 is finalized.

Proposed Solution

Once the changes in flow-go#7070 are complete, refactor MapBackData to incorporate generic type parameters for both identifiers and entities. This will involve updating the underlying map and refactoring all its associated methods (such as Add, Remove, etc.) to use generics. The refactored implementation should continue to be non-thread-safe, with concurrency management delegated to the higher-level Backend, while fully conforming to the new BackData interface.

Defenition of done

• MapBackData is refactored to use generic type parameters instead of the fixed flow.Identifier and flow.Entity.
• The BackData](https://github.com/onflow/flow-go/blob/feature/malleability/module/mempool/backData.go) functionality is implemented within MapBackData.
• All code using MapBackData is updated to work with the new generic design.
• Documentation is updated to reflect the new generic implementation.
• Tests are updated or added to ensure that the new design functions correctly without breaking existing behavior.

Related issues

#6703
#7070