XCM builder pattern (#2107)

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 <[email protected]>
Co-authored-by: command-bot <>