Move direct calls to MessageSign into new SignMessage functions in CWallet and ScriptPubKeyMan

Instead of getting a SigningProvider and then going to MessageSign,
have ScriptPubKeyMan handle the message signing internally.

src/util/message.cpp

@@ -76,3 +76,17 @@ uint256 MessageHash(const std::string& message)
 
     return hasher.GetHash();
 }
+
+std::string SigningResultString(const SigningResult res)
+{
+    switch (res) {
+        case SigningResult::OK:
+            return "No error";
+        case SigningResult::PRIVATE_KEY_NOT_AVAILABLE:
+            return "Private key not available";
+        case SigningResult::SIGNING_FAILED:
+            return "Sign failed";
+        // no default case, so the compiler can warn about missing cases
+    }
+    assert(false);
+}

src/util/message.h

@@ -39,6 +39,12 @@ enum class MessageVerificationResult {
     OK
 };
 
+enum class SigningResult {
+    OK, //!< No error
+    PRIVATE_KEY_NOT_AVAILABLE,
+    SIGNING_FAILED,
+};
+
 /** Verify a signed message.
  * @param[in] address Signer's bitcoin address, it must refer to a public key.
  * @param[in] signature The signature in base64 format.
@@ -65,4 +71,6 @@ bool MessageSign(
  */
 uint256 MessageHash(const std::string& message);
 
+std::string SigningResultString(const SigningResult res);
+
 #endif // BITCOIN_UTIL_MESSAGE_H