Add commons crate for BridgeHub runtimes [do not merge]

[vgeddes]

This PR adds a crate bridge-hub-common with common utilities for all BridgeHub runtimes (similar to the purpose of the asset-hub-common crate).

It is not meant to hold runtime config or utilities that are specific to either Snowbridge or ParityBridge. There are other crates for that. Rather, it is meant to handle cross-cutting concerns that affect all bridges on BridgeHub.

The crate initially provides specialized config for the MessageQueue pallet. See paritytech#2146 for background and discussion.

Changes:

• Introduce a AggregateMessageOrigin that identifies message queues belonging to either XCMP or to Snowbridge
• Introduce BridgeHubMessageRouter, a message processor that routes messages either to XCMP or to Snowbridge.

[yrong]

@vgeddes Seems now we have 2 different kinds of MessageOrigin.

AggregateMessageOrigin in

polkadot-sdk/cumulus/primitives/core/src/lib.rs

Line 83 in 138892d

pub enum AggregateMessageOrigin {

and the one in this PR.

Consider the change you made in Snowfork/snowbridge#1005 I would suggest without adding a new kind of AggregateMessageOrigin just add a general enum item into the previous cumulus primitive. Something like this:

#[derive(Encode, Decode, MaxEncodedLen, Clone, Eq, PartialEq, TypeInfo, Debug)]
pub enum AggregateMessageOrigin {
	/// The message came from the para-chain itself.
	Here,
	/// The message came from the relay-chain.
	///
	/// This is used by the DMP queue.
	Parent,
	/// The message came from a sibling para-chain.
	///
	/// This is used by the HRMP queue.
	Sibling(ParaId),
        /// For other customize origin like snowbridge
        GeneralKey([u8; 32])
}

@claravanstaden Seems you've merged the change here into #18 ? Then the integration tests there will not get compiled

 cargo check -p bridge-hub-rococo-integration-tests
...
error[E0277]: can't compare `bridge_hub_common::message_queue::AggregateMessageOrigin` with `CumulusAggregateMessageOrigin`
   --> cumulus/parachains/integration-tests/emulated/chains/parachains/bridges/bridge-hub-rococo/src/lib.rs:28:1
    |
28  | / decl_test_parachains! {
29  | |     pub struct BridgeHubRococo {
30  | |         genesis = genesis::genesis(),
31  | |         on_init = {
...   |
44  | |     },
45  | | }
    | |_^ no implementation for `bridge_hub_common::message_queue::AggregateMessageOrigin == CumulusAggregateMessageOrigin`

because the xcm-emulator in

polkadot-sdk/cumulus/xcm/xcm-emulator/src/lib.rs

Line 255 in 138892d

type MessageProcessor: ProcessMessage<Origin = CumulusAggregateMessageOrigin> + ServiceQueues;

accepts only the former.

And my suggestion above(i.e. stick to one AggregateMessageOrigin) will also resolve the compile issue here.