Make consensus `SlotWorker` don't assume a slot is time / duration (#7441)

* Make consensus `SlotWorker` don't assume a slot is time / duration

This removes the last bit of assumption that a slot is always `time /
duration`. This will be required by parachains where a slot will be the
relay chain block number. Besides this there are also some other drive
by changes. One more notable is that `on_slot` now returns a
`SlotResult` that holds the block and a potential storage proof.

To simplify the implementation and usage of the `SimpleSlotWorker` the
`SlotWorker` trait is now implemented for each type that implements
`SimpleSlotWorker`.

* Update client/consensus/slots/src/lib.rs

Co-authored-by: André Silva <[email protected]>

* Update client/consensus/slots/src/lib.rs

Co-authored-by: André Silva <[email protected]>

Co-authored-by: André Silva <[email protected]>