[pallet-assets] add asset_exists(id: AssetId) function (#12782)

* check if an asset exists via loose coupling

* Update frame/support/src/traits/tokens/fungibles.rs

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: parity-processbot <>

substrate/frame/support/src/traits/tokens/fungibles.rs

@@ -83,6 +83,9 @@ pub trait Inspect<AccountId> {
 		who: &AccountId,
 		amount: Self::Balance,
 	) -> WithdrawConsequence<Self::Balance>;
+
+	/// Returns `true` if an `asset` exists.
+	fn asset_exists(asset: Self::AssetId) -> bool;
 }
 
 /// Trait for reading metadata from a fungible asset.