kernel: Add notification interface

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.

Define a new kernel notification class with virtual methods for
notifying about internal kernel events. Create a new file in the node
library for defining a function creating the default set of notification
methods such that these do not need to be re-defined all over the
codebase. As a first step, add a `blockTip` method, wrapping
`uiInterface.NotifyBlockTip`.

src/bitcoin-chainstate.cpp

@@ -12,6 +12,7 @@
 // It is part of the libbitcoinkernel project.
 
 #include <kernel/chainparams.h>
+#include <kernel/chainstatemanager_opts.h>
 #include <kernel/checks.h>
 #include <kernel/context.h>
 #include <kernel/validation_cache_sizes.h>
@@ -34,6 +35,7 @@
 #include <filesystem>
 #include <functional>
 #include <iosfwd>
+#include <memory>
 
 int main(int argc, char* argv[])
 {
@@ -80,12 +82,22 @@ int main(int argc, char* argv[])
 
     GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
 
+    class KernelNotifications : public kernel::Notifications
+    {
+    public:
+        void blockTip(SynchronizationState, CBlockIndex&) override
+        {
+            std::cout << "Block tip changed" << std::endl;
+        }
+    };
+    auto notifications = std::make_unique<KernelNotifications>();
 
     // SETUP: Chainstate
     const ChainstateManager::Options chainman_opts{
         .chainparams = *chainparams,
         .datadir = gArgs.GetDataDirNet(),
         .adjusted_time_callback = NodeClock::now,
+        .notifications = *notifications,
     };
     const node::BlockManager::Options blockman_opts{
         .chainparams = chainman_opts.chainparams,