Add XCM config for Snowbridge

[alistair-singh]

Description

New feature: Integrate assets from Snowbridge.

Tasks

• [X] Allow DOT to be sent from Asset Hub as a reserve chain.
• [X] Allow bridge assets (parents > 2) to be sent from Asset Hub as a reserve chain.
• [ ] Add an Asset Transactor that will store bridged assets in pallet-assets.
• [ ] These asset ids need to be mapped to an XC-20 token.

Questions

• [ ] Are we going to allow DOT as an execution fee? We may need to implement a trader?
• [ ] Trac bridged through Snowbridge will technically be a different asset, Is making it fungible with the existing Trac?

How Has This Been Tested?

Testing still TBC.

• [ ] In Rococo, can a bridged ERC20 be sent from sepolia Ethereum to NueroWeb chain's pallet-assets?
• [ ] Can the same ERC20 be sent from NueroWeb back to Asset Hub?
• [ ] Is the ERC20 available as an XC-20 token?

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules