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 <kungfukeith11@gmail.com>
Co-authored-by: command-bot <>

prdoc/pr_2107.prdoc

@@ -0,0 +1,24 @@
+# Schema: Parity PR Documentation Schema (prdoc)
+# See doc at https://github.com/paritytech/prdoc
+
+title: Add a builder pattern to create XCM programs
+
+doc:
+  - audience: Core Dev
+    description: |
+      XCMs can now be built using a builder pattern like so:
+      Xcm::builder()
+        .withdraw_asset(assets)
+        .buy_execution(fees, weight_limit)
+        .deposit_asset(assets, beneficiary)
+        .build();
+
+migrations:
+  db: []
+
+  runtime: []
+
+crates:
+  - name: xcm
+
+host_functions: []