Disabled validators runtime API (#1257)

Exposes disabled validators list via a runtime API.

---------

Co-authored-by: ordian <noreply@reusable.software>
Co-authored-by: ordian <write@reusable.software>

polkadot/primitives/src/runtime_api.rs

@@ -248,6 +248,7 @@ sp_api::decl_runtime_apis! {
 		#[api_version(6)]
 		fn minimum_backing_votes() -> u32;
 
+
 		/***** Added in v7: Asynchronous backing *****/
 
 		/// Returns the state of parachain backing for a given para.
@@ -257,5 +258,11 @@ sp_api::decl_runtime_apis! {
 		/// Returns candidate's acceptance limitations for asynchronous backing for a relay parent.
 		#[api_version(7)]
 		fn async_backing_params() -> AsyncBackingParams;
+
+		/***** Added in v8 *****/
+
+		/// Returns a list of all disabled validators at the given block.
+		#[api_version(8)]
+		fn disabled_validators() -> Vec<ValidatorIndex>;
 	}
 }