Aura: Adds some compatibility mode to support old chains (#12492)

* Aura: Adds some compatibility mode to support old chains

In https://github.com/paritytech/substrate/pull/9132 we changed the way how we get the authorities
from the runtime. Before this mentioned pr we would call `initialize_block` before calling the
authorities runtime function. The problem with this was that when you have a block X that would
switch the authority set, it would already be signed by an authority of the new set. This was wrong,
as a block should only be signed by the current authority set. As this change is a hard fork, this
pr brings back the possibility for users that have a chain running with this old logic to upgrade.

They will need to use:
```
CompatibilityMode::UseInitializeBlock { until: some_block_in_the_future }
```

Using this compatibility mode will make the node behave like the old nodes, aka calling
`initialize_block` before doing the actual runtime call to `authorities`. Then when the given
`until` block is being build/imported the node switches to the new behaviour of not calling
`initialize_block` before. This is a hard fork, so the `until` block should be chosen wisely as a
point where all nodes in the network have upgraded.

* Fixes

* Make docs ready

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

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

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

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

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

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

* FMT

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

substrate/bin/node-template/node/src/service.rs

@@ -132,6 +132,7 @@ pub fn new_partial(
 			registry: config.prometheus_registry(),
 			check_for_equivocation: Default::default(),
 			telemetry: telemetry.as_ref().map(|x| x.handle()),
+			compatibility_mode: Default::default(),
 		})?;
 
 	Ok(sc_service::PartialComponents {
@@ -280,6 +281,7 @@ pub fn new_full(mut config: Configuration) -> Result<TaskManager, ServiceError>
 				block_proposal_slot_portion: SlotProportion::new(2f32 / 3f32),
 				max_block_proposal_slot_portion: None,
 				telemetry: telemetry.as_ref().map(|x| x.handle()),
+				compatibility_mode: Default::default(),
 			},
 		)?;