Previously, block gaps could only be created by warp sync, but with upcoming
changes (paritytech#5406), block gaps will also be generated by fast sync, thus
`BlockGapType` is needed.

This refactor converts the existing `(NumberFor<Block>, NumberFor<Block>)`
into a dedicated `BlockGap<NumberFor<Block>>` struct. This change is purely
structural and does not alter existing logic, but it lays the groundwork for
future changes.

This commit introduces changes to address compatibility issues
resulting from the transition from the old block gap representation
`(NumberFor<Block>, NumberFor<Block>)` to the new `BlockGap<NumberFor<Block>>`
structure.