Follow up from
https://github.com/paritytech/polkadot-sdk/pull/6362#issuecomment-2629744365

The goal of this PR is to have the scheduler pallet work on a parachain
which does not produce blocks on a regular schedule, thus can use the
relay chain as a block provider.

Because blocks are not produced regularly, we cannot make the assumption
that block number increases monotonically, and thus have new logic to
handle multiple spend periods passing between blocks.

Requirement: 

instead of using the hard coded system block number. We add an
associated type BlockNumberProvider

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>