Add GetEncryptionKey() and HasEncryptionKeys() to WalletStorage

Adds functions in WalletStorage that allow ScriptPubKeyMans to check
and get encryption keys from the wallet.

src/wallet/wallet.cpp

@@ -4056,3 +4056,13 @@ LegacyScriptPubKeyMan* CWallet::GetLegacyScriptPubKeyMan() const
 {
     return m_spk_man.get();
 }
+
+const CKeyingMaterial& CWallet::GetEncryptionKey() const
+{
+    return vMasterKey;
+}
+
+bool CWallet::HasEncryptionKeys() const
+{
+    return !mapMasterKeys.empty();
+}