Custom barrier for DescendOrigin

[Dinonard]

Pull Request Summary

Adds custom Barrier for XCM that allows sequence like:

DescendOrigin -> WithdrawAsset -> BuyExecution

This is required if we want to support remote execution from non-parachain accounts. The common sense here is the assumption that none of the parachains would allow sending of customizable XCM sequence that isn't prepended with DescendOrigin.

Check list

• [x] added unit tests
• [x] updated documentation

[github-actions[bot]]

Package	Line Rate	Branch Rate	Health
frame/xc-asset-config/src	70%	0%	➖
precompiles/utils/macro/tests	0%	0%	❌
precompiles/utils/src	72%	0%	➖
precompiles/substrate-ecdsa/src	78%	0%	➖
chain-extensions/types/rmrk/src	0%	0%	❌
chain-extensions/types/xvm/src	0%	0%	❌
precompiles/assets-erc20/src	73%	0%	➖
chain-extensions/rmrk/src	0%	0%	❌
chain-extensions/xvm/src	0%	0%	❌
frame/collator-selection/src	80%	0%	✔
chain-extensions/dapps-staking/src	0%	0%	❌
frame/pallet-xvm/src	11%	0%	❌
frame/pallet-xcm/src	65%	0%	➖
frame/pallet-xvm/src/pallet	34%	0%	❌
frame/custom-signatures/src	57%	0%	➖
precompiles/xcm/src	84%	0%	✔
precompiles/xvm/src	94%	0%	✔
precompiles/utils/src/data	72%	0%	➖
precompiles/utils/macro/src	0%	0%	❌
precompiles/sr25519/src	79%	0%	➖
frame/block-reward/src	85%	0%	✔
primitives/xcm/src	66%	0%	➖
frame/dapps-staking/src	83%	0%	✔
frame/dapps-staking/src/pallet	90%	0%	✔
precompiles/dapps-staking/src	93%	0%	✔
chain-extensions/types/dapps-staking/src	0%	0%	❌
Summary	60% (2593 / 4352)	0% (0 / 0)	➖

Minimum allowed line rate is 50%

[Dinonard]

One question though: why do we consider a sequence as valid if it contains some extra instructions? I mean, technically we can, of course, but should we?

Because the point of this barrier is to ensure that whoever is sending the sequence pays for it's execution. Whatever they do after paying is fine - that's why the UT contains scenario with one extra instruction.

The sequence being matched in the code is useless by itself.