make SelfParaId a metadata constant (#3517)

expose para id via metadata

#2116 is blocked by this

cumulus/pallets/parachain-system/src/lib.rs

@@ -205,6 +205,7 @@ pub mod pallet {
 		type OnSystemEvent: OnSystemEvent;
 
 		/// Returns the parachain ID we are running with.
+		#[pallet::constant]
 		type SelfParaId: Get<ParaId>;
 
 		/// The place where outbound XCMP messages come from. This is queried in `finalize_block`.