wallet: Write new descriptor's cache in AddWalletDescriptor

If a new WalletDescriptor is provided to us with a cache, write the
cache to disk as well.

Author: achow101

src/wallet/wallet.cpp

@@ -3748,6 +3748,10 @@ util::Result<std::reference_wrapper<DescriptorScriptPubKeyMan>> CWallet::AddWall
         // Save the descriptor to memory
         uint256 id = new_spk_man->GetID();
         AddScriptPubKeyMan(id, std::move(new_spk_man));
+
+        // Write the existing cache to disk
+        WalletBatch batch(GetDatabase());
+        batch.WriteDescriptorCacheItems(id, desc.cache);
     }
 
     // Add the private keys to the descriptor