Move GetSpendingKeyForPaymentAddress visitor into wallet.h

Also fixes it to not use the global pwalletMain.

src/wallet/wallet.cpp

@@ -4373,3 +4373,37 @@ bool PaymentAddressBelongsToWallet::operator()(const libzcash::InvalidEncoding&
 {
     return false;
 }
+
+boost::optional<libzcash::SpendingKey> GetSpendingKeyForPaymentAddress::operator()(
+    const libzcash::SproutPaymentAddress &zaddr) const
+{
+    libzcash::SproutSpendingKey k;
+    if (m_wallet->GetSproutSpendingKey(zaddr, k)) {
+        return libzcash::SpendingKey(k);
+    } else {
+        return boost::none;
+    }
+}
+
+boost::optional<libzcash::SpendingKey> GetSpendingKeyForPaymentAddress::operator()(
+    const libzcash::SaplingPaymentAddress &zaddr) const
+{
+    libzcash::SaplingIncomingViewingKey ivk;
+    libzcash::SaplingFullViewingKey fvk;
+    libzcash::SaplingSpendingKey sk;
+
+    if (m_wallet->GetSaplingIncomingViewingKey(zaddr, ivk) &&
+        m_wallet->GetSaplingFullViewingKey(ivk, fvk) &&
+        m_wallet->GetSaplingSpendingKey(fvk, sk)) {
+        return libzcash::SpendingKey(sk);
+    } else {
+        return boost::none;
+    }
+}
+
+boost::optional<libzcash::SpendingKey> GetSpendingKeyForPaymentAddress::operator()(
+    const libzcash::InvalidEncoding& no) const
+{
+    // Defaults to InvalidEncoding
+    return libzcash::SpendingKey();
+}