pallet-aura: add feature-flagged explicit slot duration type (#14649)

* aura: add feature-flagged explicit slot duration type

* fmt

* add some comments

* have node-template use new explicit feature

* fix mock

* fmt

* use the experimental feature flag instead

* checkout master Cargo.lock

substrate/bin/node-template/runtime/src/lib.rs

@@ -207,6 +207,9 @@ impl pallet_aura::Config for Runtime {
 	type DisabledValidators = ();
 	type MaxAuthorities = ConstU32<32>;
 	type AllowMultipleBlocksPerSlot = ConstBool<false>;
+
+	#[cfg(feature = "experimental")]
+	type SlotDuration = pallet_aura::MinimumPeriodTimesTwo<Runtime>;
 }
 
 impl pallet_grandpa::Config for Runtime {