Create BCLog::Logger::LogLevelsString() helper function

Co-authored-by: "Jon Atack <jon@atack.com>"

src/logging.h

@@ -186,6 +186,9 @@ namespace BCLog {
             return Join(LogCategoriesList(), ", ", [&](const LogCategory& i) { return i.category; });
         };
 
+        //! Returns a string with all user-selectable log levels.
+        std::string LogLevelsString() const;
+
         bool DefaultShrinkDebugFile() const;
     };