Migrate pallet-im-online to pallet attribute macro. (paritytech#8714)

* Migrate pallet-im-online to pallet attribute macro.

* Move validate_unsigned into pallet macro.

* fix metadata

* fix test

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

Author: shaunxw

frame/im-online/src/benchmarking.rs

@@ -29,7 +29,7 @@ use sp_runtime::traits::{ValidateUnsigned, Zero};
 use sp_runtime::transaction_validity::TransactionSource;
 use frame_support::traits::UnfilteredDispatchable;
 
-use crate::Module as ImOnline;
+use crate::Pallet as ImOnline;
 
 const MAX_KEYS: u32 = 1000;
 const MAX_EXTERNAL_ADDRESSES: u32 = 100;