add appchain agent message delivery

[kb1ns]

No description provided.

[riversyang]

Actually, I don't understand the intention of an account to sign a message for another account. I assume you want to operate the scenario like interchain account. In that case, the param sender_id_in_near of function deliver_appchain_agent_message should be removed, and you can use env::predecessor_account_id() as sender_id_in_near in corresponding AppchainNotification.

Even if it is the case I just mentioned, I still confused about the intention of this AppchainNotification. As the appchain anchor contract is a protocol level implementation of Octopus cross-chain bridge, I don't think we should add a type of Trojan horse message to our cross-chain message design. The bridge do not understand the intention of the new AppchainNotification, so it can not be handled/recovered properly if something goes wrong.

Furthermore, the appchain anchor contract can not work alone, it also needs a specific pallet in Octopus Appchain template to support the new AppchainNotification. Is there a pr on substrate side which can help us to understand the intention of appchain agent message?