Use SignedAmount in GetTransactionDetail

Turns out this amount is signed, use `SignedAmount` instead of `Amount`.

json/src/model/wallet.rs

@@ -107,7 +107,8 @@ pub struct GetTransaction {
 pub struct GetTransactionDetail {
     pub address: Address<NetworkUnchecked>,
     pub category: GetTransactionDetailCategory,
-    pub amount: Amount,
+    #[serde(default, with = "bitcoin::amount::serde::as_btc")]
+    pub amount: SignedAmount,
     pub label: Option<String>,
     pub vout: u32,
     #[serde(default, with = "bitcoin::amount::serde::as_btc::opt")]

json/src/v17/wallet/convert.rs

@@ -142,7 +142,7 @@ impl TryFrom<v17::GetTransactionDetail> for model::GetTransactionDetail {
         use GetTransactionDetailError as E;
 
         let address = Address::from_str(&json.address).map_err(E::Address)?;
-        let amount = Amount::from_btc(json.amount).map_err(E::Amount)?;
+        let amount = SignedAmount::from_btc(json.amount).map_err(E::Amount)?;
         // FIMXE: Use combinators.
         let fee = match json.fee {
             None => None,