Different XCM builders, default one requires fee payment (#2253)

Adding on top of the new builder pattern for creating XCM programs, I'm
adding some more APIs:

```rust
let paying_fees: Xcm<()> = Xcm::builder() // Only allow paying for fees
  .withdraw_asset() // First instruction has to load the holding register
  .buy_execution() // Second instruction has to be `buy_execution`
  .build();

let paying_fees_invalid: Xcm<()> = Xcm::builder()
  .withdraw_asset()
  .build(); // Invalid, need to pay for fees

let not_paying_fees: Xcm<()> = Xcm::builder_unpaid()
  .unpaid_execution() // Needed
  .withdraw_asset()
  .deposit_asset()
  .build();

let all_goes: Xcm<()> = Xcm::builder_unsafe() // You can do anything
  .withdraw_asset()
  .deposit_asset()
  .build();
```

The invalid bits are because the methods don't even exist on the types
that you'd want to call them on.

---------

Co-authored-by: command-bot <>

polkadot/xcm/procedural/tests/ui/builder_pattern/unexpected_attribute.rs

@@ -0,0 +1,32 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Test error when using wrong attribute.
+
+use xcm_procedural::Builder;
+
+struct Xcm<Call>(pub Vec<Instruction<Call>>);
+
+#[derive(Builder)]
+enum Instruction<Call> {
+    #[builder(funds_holding)]
+    WithdrawAsset(u128),
+    BuyExecution { fees: u128 },
+    UnpaidExecution { weight_limit: (u32, u32) },
+    Transact { call: Call },
+}
+
+fn main() {}