XCM messages for channel management between parachains

[pepyakin]

We should introduce a set of XCM messages tailored for opening/accepting/closing channels between parachains. These messages are meant to be interpreted by the relay-chain and sent by a parachain.

Right now channel management is possible but is rather involved (see here).

[samelamin]

This would be awesome

I think an ideal UX would be for a parachain a to call an dispatchable that will send out a request to parachain B

Then parachain B picks up the request and sends out an accepted message

This also could be a great reference to other parachains who want to build functionality based on xcmp - like us! - and would give them a great starting point

Really you just want a simple way to open an hrmp channel between parachains

Also as a side note, polkadot launch doesn't seem to open the channels that are defined but that might be because I'm doing something wrong

[4meta5]

#2661