wallet: Introduce a callback called after TopUp completes

After TopUp completes, the wallet containing each SPKM will want to know
what new scriptPubKeys were generated. In order for all TopUp calls
(including ones internal the the SPKM), we use a callback function in
the WalletStorage interface.

src/wallet/wallet.cpp

@@ -4334,4 +4334,14 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
     }
     return res;
 }
+
+void CWallet::CacheNewScriptPubKeys(const std::set<CScript>& spks, ScriptPubKeyMan* spkm)
+{
+}
+
+void CWallet::TopUpCallback(const std::set<CScript>& spks, ScriptPubKeyMan* spkm)
+{
+    // Update scriptPubKey cache
+    CacheNewScriptPubKeys(spks, spkm);
+}
 } // namespace wallet