Add claim_assets extrinsic to pallet-xcm (#3403)

If an XCM execution fails or ends with leftover assets, these will be
trapped.
In order to claim them, a custom XCM has to be executed, with the
`ClaimAsset` instruction.
However, arbitrary XCM execution is not allowed everywhere yet and XCM
itself is still not easy enough to use for users out there with trapped
assets.
This new extrinsic in `pallet-xcm` will allow these users to easily
claim their assets, without concerning themselves with writing arbitrary
XCMs.

Part of fixing https://github.com/paritytech/polkadot-sdk/issues/3495

---------

Co-authored-by: command-bot <>
Co-authored-by: Adrian Catangiu <adrian@parity.io>

cumulus/parachains/runtimes/contracts/contracts-rococo/src/lib.rs

@@ -764,6 +764,13 @@ impl_runtime_apis! {
 						dest
 					)
 				}
+
+				fn get_asset() -> Asset {
+					Asset {
+						id: AssetId(Location::parent()),
+						fun: Fungible(EXISTENTIAL_DEPOSIT),
+					}
+				}
 			}
 
 			let whitelist: Vec<TrackedStorageKey> = vec![