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/xcm-simulator/example/src/lib.rs

@@ -649,23 +649,4 @@ mod tests {
 			);
 		});
 	}
-
-	#[test]
-	fn builder_pattern_works() {
-		let asset: MultiAsset = (Here, 100u128).into();
-		let beneficiary: MultiLocation = AccountId32 { id: [0u8; 32], network: None }.into();
-		let message: Xcm<()> = Xcm::builder()
-			.withdraw_asset(asset.clone().into())
-			.buy_execution(asset.clone(), Unlimited)
-			.deposit_asset(asset.clone().into(), beneficiary)
-			.build();
-		assert_eq!(
-			message,
-			Xcm(vec![
-				WithdrawAsset(asset.clone().into()),
-				BuyExecution { fees: asset.clone(), weight_limit: Unlimited },
-				DepositAsset { assets: asset.into(), beneficiary },
-			])
-		);
-	}
 }