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/node/kernel_notifications.h

@@ -23,6 +23,8 @@ public:
     void headerTip(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) override;
 
     void progress(const bilingual_str& title, int progress_percent, bool resume_possible) override;
+
+    void warning(const bilingual_str& warning) override;
 };
 } // namespace node