Add basic XCM configuration

[svyatonik]

The fully working bridge may be started by using branches, mentioned in https://github.com/paritytech/parity-bridges-common/issues/2547. I'm going to open multiple PRs to make it easier to review - some PRs need a different set of reviewers

From bridges perspective, we need an XCM configuration to be able to dispatch inbound messages. I'm not pretending to be an XCM expert, so I'm opening this PR to ask questions and collectively understand what we need. This PR doesn't do anything with bridge - it just adds the xcm_executor::Config, required to be able to execute XCM programs.

Some important points:

• I haven't added the pallet-xcm to runtime, because IIUC we don't need it here. However if you think we need it, we may try to plug it in;
• I've mentioned Kawabunga parachain - it is the parachain, which will be bridging with the Bulletin chain over Polkadot BH, which is not available;
• I'll leave explicit PR comments where I need some help and/or we may need to discuss something.

Also pinging https://github.com/orgs/paritytech/teams/bridges-core for review

[svyatonik]

@zdave-parity Could you, please, approve if you have no more comments. Or else give one of reviewers a write access to this repo. We can't merge without that. Thank you!