ed25519_verify: Support using dalek for historical blocks (#12661)

* ed25519_verify: Support using dalek for historical blocks

The switch from `ed25519-dalek` to `ed25519-zebra` was actually a breaking change. `ed25519-zebra`
is more permissive. To support historical blocks when syncing a chain this pull request introduces
an externalities extension `UseDalekExt`. This extension is just used as a signaling mechanism to
`ed25519_verify` to use `ed25519-dalek` when it is present. Together with `ExtensionBeforeBlock` it
can be used to setup a node in way to sync historical blocks that require `ed25519-dalek`, because
they included a transaction that verified differently as when using `ed25519-zebra`.

This feature can be enabled in the following way. In the chain service file, directly after the
client is created, the following code should be added:

```
use sc_client_api::ExecutorProvider;
client.execution_extensions().set_extensions_factory(
	sc_client_api::execution_extensions::ExtensionBeforeBlock::<Block, sp_io::UseDalekExt>::new(BLOCK_NUMBER_UNTIL_DALEK_SHOULD_BE_USED)
);
```

* Fix doc

* More fixes

* Update client/api/src/execution_extensions.rs

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

* Fix merge and warning

* Fix docs

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

substrate/test-utils/client/src/lib.rs

@@ -229,11 +229,6 @@ impl<Block: BlockT, ExecutorDispatch, Backend, G: GenesisInit>
 			&storage,
 			self.fork_blocks,
 			self.bad_blocks,
-			ExecutionExtensions::new(
-				self.execution_strategies,
-				self.keystore,
-				sc_offchain::OffchainDb::factory_from_backend(&*self.backend),
-			),
 			None,
 			None,
 			ClientConfig {
@@ -285,6 +280,11 @@ impl<Block: BlockT, D, Backend, G: GenesisInit>
 			executor,
 			Box::new(sp_core::testing::TaskExecutor::new()),
 			Default::default(),
+			ExecutionExtensions::new(
+				self.execution_strategies.clone(),
+				self.keystore.clone(),
+				sc_offchain::OffchainDb::factory_from_backend(&*self.backend),
+			),
 		)
 		.expect("Creates LocalCallExecutor");