kernel: Add headerTip 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.

src/bitcoin-chainstate.cpp

@@ -32,6 +32,7 @@
 #include <validationinterface.h>
 
 #include <cassert>
+#include <cstdint>
 #include <filesystem>
 #include <functional>
 #include <iosfwd>
@@ -89,6 +90,10 @@ int main(int argc, char* argv[])
         {
             std::cout << "Block tip changed" << std::endl;
         }
+        void headerTip(SynchronizationState, int64_t height, int64_t timestamp, bool presync) override
+        {
+            std::cout << "Header tip changed: " << height << ", " << timestamp << ", " << presync << std::endl;
+        }
     };
     auto notifications = std::make_unique<KernelNotifications>();