Implement the #[sv::custom(query=CustomQuery)] attribute on the contract level

[jawoznia]

Part of #16

• [x] #[sv::custom(query=CustomQuery)] causes generated entry points and messages expect Deps<CustomQuery>
• [x] Contract MT helper implements Contract<.., CustomQuery>
• [x] All MT helpers generated work properly with generic App (with specified QueryC)
• [x] #[messages(...: custom(query))] on the contract implementation causes the conversion of deps dispatched to the interface