Contracts V2

[vgeddes]

Tasks:

• [x] V2 types
• [x] V2 inbound messaging
• [x] V2 outbound messaging
• [x] Refactor to support both V1 & V2
• [x] Re-implement native token registration for V2 & Kusama compatibility
• [ ] Generate encoded XCM fragments for token registration
• [ ] Split IGateway into IGatewayV1 and IGatewayV2
• [ ] Remove duplicated error definitions scattered throughout code
• [ ] Unit tests
• [ ] Code documentation