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/interfaces/wallet.cpp

@@ -132,6 +132,10 @@ public:
         }
         return false;
     }
+    SigningResult signMessage(const std::string& message, const PKHash& pkhash, std::string& str_sig) override
+    {
+        return m_wallet->SignMessage(message, pkhash, str_sig);
+    }
     bool isSpendable(const CTxDestination& dest) override { return m_wallet->IsMine(dest) & ISMINE_SPENDABLE; }
     bool haveWatchOnly() override
     {

src/interfaces/wallet.h

@@ -10,6 +10,7 @@
 #include <script/standard.h>           // For CTxDestination
 #include <support/allocators/secure.h> // For SecureString
 #include <ui_interface.h>              // For ChangeType
+#include <util/message.h>
 
 #include <functional>
 #include <map>
@@ -87,6 +88,9 @@ public:
     //! Get private key.
     virtual bool getPrivKey(const CScript& script, const CKeyID& address, CKey& key) = 0;
 
+    //! Sign message
+    virtual SigningResult signMessage(const std::string& message, const PKHash& pkhash, std::string& str_sig) = 0;
+
     //! Return whether wallet has private key.
     virtual bool isSpendable(const CTxDestination& dest) = 0;