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/init.cpp

@@ -45,6 +45,7 @@
 #include <node/chainstatemanager_args.h>
 #include <node/context.h>
 #include <node/interface_ui.h>
+#include <node/kernel_notifications.h>
 #include <node/mempool_args.h>
 #include <node/mempool_persist_args.h>
 #include <node/miner.h>
@@ -124,6 +125,7 @@ using node::DEFAULT_PERSIST_MEMPOOL;
 using node::DEFAULT_PRINTPRIORITY;
 using node::fReindex;
 using node::g_indexes_ready_to_sync;
+using node::KernelNotifications;
 using node::LoadChainstate;
 using node::MempoolPath;
 using node::NodeContext;
@@ -1019,9 +1021,11 @@ bool AppInitParameterInteraction(const ArgsManager& args, bool use_syscall_sandb
 
     // Also report errors from parsing before daemonization
     {
+        KernelNotifications notifications{};
         ChainstateManager::Options chainman_opts_dummy{
             .chainparams = chainparams,
             .datadir = args.GetDataDirNet(),
+            .notifications = notifications,
         };
         if (const auto error{ApplyArgsManOptions(args, chainman_opts_dummy)}) {
             return InitError(*error);
@@ -1427,12 +1431,14 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
     // ********************************************************* Step 7: load block chain
 
+    node.notifications = std::make_unique<KernelNotifications>();
     fReindex = args.GetBoolArg("-reindex", false);
     bool fReindexChainState = args.GetBoolArg("-reindex-chainstate", false);
     ChainstateManager::Options chainman_opts{
         .chainparams = chainparams,
         .datadir = args.GetDataDirNet(),
         .adjusted_time_callback = GetAdjustedTime,
+        .notifications = *node.notifications,
     };
     Assert(!ApplyArgsManOptions(args, chainman_opts)); // no error can happen, already checked in AppInitParameterInteraction