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 <123550+andresilva@users.noreply.github.com>

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

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/primitives/consensus/common/src/lib.rs

@@ -123,6 +123,13 @@ impl RecordProof {
 	}
 }
 
+/// Will return [`RecordProof::No`] as default value.
+impl Default for RecordProof {
+	fn default() -> Self {
+		Self::No
+	}
+}
+
 impl From<bool> for RecordProof {
 	fn from(val: bool) -> Self {
 		if val {