Make sure we support dispatching messages directly at bridge hub

[svyatonik]

There are some use cases for dispatching message at the target bridge hub. So it's one hop less than our regular path (source_chain -> source_bridge_hub -> target_bridge_hub -> target_chain - assume that target_bridge_hub is the target_chain). We need to support in in v2:

• the pallet-xcm-bridge-hub implements MessageDispatch trait. Right now (afaik) it uses the xcm_builder::BridgeBlobDispatcher, which calls the send_xcm. We need to be sure that either send_xcm supports sending to the same chain or we need to use some other dispatcher;
• we need some process to open the bridge (open_bridge call of the pallet-xcm-bridge-hub) at the target BH. Relay chain governance?