Fix safe withdrawal fee type

[zed-wong]

> mixin-cli -f ~/Documents/bot.json http /safe/assets/c94ac88f-4671-3976-b60a-09064f1811e8/fees
[
  {
    "amount": "0.01",
    "asset_id": "c94ac88f-4671-3976-b60a-09064f1811e8",
    "priority": 100,
    "type": "withdrawal_fee"
  },
  {
    "amount": "0.001",
    "asset_id": "43d61dcd-e413-450d-80b8-101d5e903357",
    "priority": 10,
    "type": "withdrawal_fee"
  }
]

Add priority field to SafeWithdrawalFee interface