Add hrmp general admin track manipulator

[girazoki]

What does it do?

Adds the hrmp-manage extrinsic to xcm-transactor pallet that is able to filter on distinct origin to any other extrinsic in the pallet. The idea is that thanks to OpenGov we can have an origin capable of opening HRMP channels.

The extrinsic works as follows: We have the HRMP pallet relay encoding hardcoded in the runtime. Through the extrinsic one can choose to perform three main actions:

• InitChannel
• Accept
• Close

Based on the option selected the extrinsic will be able to encode differently. The idea is that we send 3 instrucions to the relay:

• WithdrawAsset
• BuyExecution
• SetAppendix(RefundSurplus, DepositAsset(Sov account))
• Transact

The setAppendix instruction is critical: it basically determines that no matter whether the xcm execution goes right or wong, any remaining tokens are deposited back to the sovereign account. This is important because a malicious referenda might try to withdraw more assets than what are needed to open/accept/close the HRMP channel. If we do not return them to the sovereign account, this might be short on assets for future token transfers.

However, the setAppendix is only placed after the BuyExecution instruction (otherwise it wouldnt pass relay barriers). An error in BuyExecution would not trigger the appendix. For instance, an error when trying to buy a high amount of weight would not with not-enough assets would not return the assets back. Luckily, we know the relay barriers just try to buy the amount of weight necessary to execute the message (https://github.com/paritytech/polkadot/blob/6a5d31fcbdd3ac5dd880e0bf3616e303b5bb480f/xcm/xcm-builder/src/barriers.rs#L83).

To further prevent situations in which tokens are not returned to the sovereign account, we also limit the amount that can be used as fee.

What important points reviewers should know?

Is there something left for follow-up PRs?

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?

[girazoki]

@crystalin I added a general admin track but we will probably need to modify parameters. Let me know about those

[crystalin]

@girazoki is the fee/second value manageable for with the admin track too ?

[girazoki]

@girazoki is the fee/second value manageable for with the admin track too ?

You mean if I can make the generalAdmin be responsible for changing the feePerSecond? Sure, there should be no problem with that

[librelois]

I left some minor comments, but overall it's very good