`dry_run_xcm` support for BridgeHubs

[bkontur]

Basically, the source can dry run some scenario, which results as a xcm with ExportMessage instruction, so we should be able to dry run this XCM on BridgeHub.

Some hints:

• implement InspectMessageQueues for pallet_xcm_bridge_hub or pallet-bridge-messages to get messages from OutboundLanes where the OutboundPayload is encoded BridgeMessage

• then add the instance to the tuple with xcm_config::Router on the bridge hub:

  fn dry_run_xcm(origin_location: VersionedLocation, xcm: VersionedXcm<RuntimeCall>) -> Result<XcmDryRunEffects<RuntimeEvent>, XcmDryRunApiError> {
     PolkadotXcm::dry_run_xcm::<Runtime, (xcm_config::XcmRouter, XcmOverBridgeHubWestend), RuntimeCall,    xcm_config::XcmConfig>(origin_location, xcm)
  }
  or
  
  fn dry_run_xcm(origin_location: VersionedLocation, xcm: VersionedXcm<RuntimeCall>) -> Result<XcmDryRunEffects<RuntimeEvent>, XcmDryRunApiError> {
     PolkadotXcm::dry_run_xcm::<Runtime, (xcm_config::XcmRouter, BridgeWestendMessages), RuntimeCall,    xcm_config::XcmConfig>(origin_location, xcm)
  }

• dry_xcm_run on the bridged BridgeHub is quesionable how to do, because there could possibly be DescendOrigin added here when dispatching
  • I think with Chopsticks we have the same problem
• what about Ethereum bridge?