refactor: Block unsafe fs::path std::string conversion calls

There is no change in behavior. This just helps prepare for the
transition from boost::filesystem to std::filesystem by avoiding calls
to methods which will be unsafe after the transaction to std::filesystem
to due lack of a boost::filesystem::path::imbue equivalent and inability
to set a predictable locale.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
Co-authored-by: Kiminuo <kiminuo@protonmail.com>
Co-authored-by: MarcoFalke <falke.marco@gmail.com>

src/wallet/interfaces.cpp

@@ -551,13 +551,13 @@ public:
     }
     std::string getWalletDir() override
     {
-        return GetWalletDir().string();
+        return fs::PathToString(GetWalletDir());
     }
     std::vector<std::string> listWalletDir() override
     {
         std::vector<std::string> paths;
         for (auto& path : ListDatabases(GetWalletDir())) {
-            paths.push_back(path.string());
+            paths.push_back(fs::PathToString(path));
         }
         return paths;
     }