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/test/validation_chainstatemanager_tests.cpp

@@ -4,6 +4,7 @@
 //
 #include <chainparams.h>
 #include <consensus/validation.h>
+#include <node/kernel_notifications.h>
 #include <node/utxo_snapshot.h>
 #include <random.h>
 #include <rpc/blockchain.h>
@@ -23,6 +24,7 @@
 #include <boost/test/unit_test.hpp>
 
 using node::BlockManager;
+using node::KernelNotifications;
 using node::SnapshotMetadata;
 
 BOOST_FIXTURE_TEST_SUITE(validation_chainstatemanager_tests, ChainTestingSetup)
@@ -377,10 +379,12 @@ struct SnapshotTestSetup : TestChain100Setup {
             LOCK(::cs_main);
             chainman.ResetChainstates();
             BOOST_CHECK_EQUAL(chainman.GetAll().size(), 0);
+            m_node.notifications = std::make_unique<KernelNotifications>();
             const ChainstateManager::Options chainman_opts{
                 .chainparams = ::Params(),
                 .datadir = m_args.GetDataDirNet(),
                 .adjusted_time_callback = GetAdjustedTime,
+                .notifications = *m_node.notifications,
             };
             const BlockManager::Options blockman_opts{
                 .chainparams = chainman_opts.chainparams,