RFC: add complex asset transfers instruction

[acatangiu]

This is a continuation of #54 where I lost write-access after the migration to the polkadot-fellows org.

Summary

This RFC proposes a new instruction that provide a way to initiate asset transfers which transfer multiple
types (teleports, local-reserve, destination-reserve) of assets, on remote chains using XCM alone.

The currently existing instructions are too opinionated and force each XCM asset transfer to a single
transfer type (teleport, local-reserve, destination-reserve). This results in inability to combine different
types of transfers in single transfer which results in overall poor UX when trying to move assets across
chains.

Example usage

    // pay remote fees with ROCs
    let remote_fees = Some(
        AssetTransferFilter::ReserveWithdraw(
            AssetFilter::Wild(AllOf { id: rocs_id.into(), fun: WildFungibility::Fungible })
        )
    );
    // XCM to be executed locally
    let xcm = Xcm::<penpal_runtime::RuntimeCall>(vec![
        // Withdraw both ROCs and PENs from origin account
        WithdrawAsset(assets.clone().into()),
        // Execute the transfers while paying remote fees with ROCs
        InitiateAssetsTransfer {
            dest: local_asset_hub,
            assets: vec![
                // ROCs are reserve-withdrawn on AHR
                ReserveWithdraw(rocs.into()),
                // PENs are teleported to AHR
                Teleport(pens.into()),
            ],
            remote_fees,
            remote_xcm: xcm_on_ahr,
        },
    ]);

Proof-of-Concept

I also have a proof-of-concept implementation and a proof-of-concept asset transfer test, transferring 2 different types of assets across 3 chains.

[acatangiu]

Closing in favor of polkadot-fellows/RFCs#100