kernel: Add warning method to notifications

This commit is part of the libbitcoinkernel project and seeks to remove
the ChainstateManager's and, more generally, the kernel library's
dependency on interface_ui with options methods in this and the following
few commits. By removing interface_ui from the kernel library, its
dependency on boost is reduced to just boost::multi_index.

The DoWarning and AlertNotify functions are moved out of the
validation.cpp file, which removes its dependency on interface_ui as
well as util/system.

src/bitcoin-chainstate.cpp

@@ -99,6 +99,10 @@ int main(int argc, char* argv[])
         {
             std::cout << "Progress: " << title.original << ", " << progress_percent << ", " << resume_possible << std::endl;
         }
+        void warning(const bilingual_str& warning) override
+        {
+            std::cout << "Warning: " << warning.original << std::endl;
+        }
     };
     auto notifications = std::make_unique<KernelNotifications>();