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/node/kernel_notifications.cpp

@@ -0,0 +1,16 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <node/kernel_notifications.h>
+
+#include <node/interface_ui.h>
+
+namespace node {
+
+void KernelNotifications::blockTip(SynchronizationState state, CBlockIndex& index)
+{
+    uiInterface.NotifyBlockTip(state, &index);
+}
+
+} // namespace node