Introduce `BlockGap` (#5592)

Previously, block gaps could only be created by warp sync, but block
gaps will also be generated by fast sync once #5406 is fixed. This PR is
part 1 of the detailed implementation plan in
https://github.com/paritytech/polkadot-sdk/issues/5406#issuecomment-2325064863:
refactor `BlockGap`.

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
lays the groundwork for the follow-up PR.

The compatibility concern caused by the new structure is addressed in
the second commit.

cc @dmitry-markin



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Co-authored-by: Bastian Köcher <[email protected]>