timestamps verification: verify against relay slots (#2221)

Verify timestamps against relay chain slot numbers via `OnTimestampSet`.

Closes  #2219 

# Discussion

Solution proposed in story above

# Checklist
- [ ] Updated Pallet Readme?
- [ ] Updated js/api-augment for Custom RPC APIs?
- [ ] Design doc(s) updated?
- [ ] Unit Tests added?
- [ ] e2e Tests added?
- [ ] Benchmarks added?
- [x] Spec version incremented?

runtime/frequency/src/lib.rs

@@ -404,7 +404,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("frequency"),
 	impl_name: create_runtime_str!("frequency"),
 	authoring_version: 1,
-	spec_version: 134,
+	spec_version: 135,
 	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
 	transaction_version: 1,
@@ -418,7 +418,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("frequency-testnet"),
 	impl_name: create_runtime_str!("frequency"),
 	authoring_version: 1,
-	spec_version: 134,
+	spec_version: 135,
 	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
 	transaction_version: 1,
@@ -657,6 +657,9 @@ impl pallet_time_release::Config for Runtime {
 impl pallet_timestamp::Config for Runtime {
 	/// A timestamp: milliseconds since the unix epoch.
 	type Moment = u64;
+	#[cfg(not(feature = "frequency-no-relay"))]
+	type OnTimestampSet = Aura;
+	#[cfg(feature = "frequency-no-relay")]
 	type OnTimestampSet = ();
 	type MinimumPeriod = MinimumPeriod;
 	type WeightInfo = weights::pallet_timestamp::SubstrateWeight<Runtime>;