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/util/setup_common.cpp

@@ -23,6 +23,7 @@
 #include <node/blockstorage.h>
 #include <node/chainstate.h>
 #include <node/context.h>
+#include <node/kernel_notifications.h>
 #include <node/mempool_args.h>
 #include <node/miner.h>
 #include <node/validation_cache_args.h>
@@ -64,6 +65,7 @@ using node::ApplyArgsManOptions;
 using node::BlockAssembler;
 using node::BlockManager;
 using node::CalculateCacheSizes;
+using node::KernelNotifications;
 using node::LoadChainstate;
 using node::RegenerateCommitments;
 using node::VerifyLoadedChainstate;
@@ -182,11 +184,14 @@ ChainTestingSetup::ChainTestingSetup(const ChainType chainType, const std::vecto
 
     m_cache_sizes = CalculateCacheSizes(m_args);
 
+    m_node.notifications = std::make_unique<KernelNotifications>();
+
     const ChainstateManager::Options chainman_opts{
         .chainparams = chainparams,
         .datadir = m_args.GetDataDirNet(),
         .adjusted_time_callback = GetAdjustedTime,
         .check_block_index = true,
+        .notifications = *m_node.notifications,
     };
     const BlockManager::Options blockman_opts{
         .chainparams = chainman_opts.chainparams,