Added a proc macro to be able to write XCMs using the builder pattern.
This means we go from having to do this:
```rust
let message: Xcm<()> = Xcm(vec![
WithdrawAsset(assets),
BuyExecution { fees: asset, weight_limit: Unlimited },
DepositAsset { assets, beneficiary },
]);
```
to this:
```rust
let message: Xcm<()> = Xcm::builder()
.withdraw_asset(assets)
.buy_execution(asset, Unlimited),
.deposit_asset(assets, beneficiary)
.build();
```
---------
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>
Co-authored-by: command-bot <>
Francisco Aguirre