kernel: Add notifications context option to C header

The notifications are used for notifying on connected blocks and on
warning and fatal error conditions.

The user of the C header may define callbacks that gets passed to the
internal notification object in the
`kernel_NotificationInterfaceCallbacks` struct.

Each of the callbacks take a `user_data` argument that gets populated
from the `user_data` value in the struct. It can be used to recreate the
structure containing the callbacks on the user's side, or to give the
callbacks additional contextual information.

src/test/kernel/test_kernel.cpp

@@ -60,6 +60,35 @@ public:
     }
 };
 
+class TestKernelNotifications : public KernelNotifications
+{
+public:
+    void HeaderTipHandler(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) override
+    {
+        BOOST_CHECK_GT(timestamp, 0);
+    }
+
+    void WarningSetHandler(Warning warning, std::string_view message) override
+    {
+        std::cout << "Kernel warning is set: " << message << std::endl;
+    }
+
+    void WarningUnsetHandler(Warning warning) override
+    {
+        std::cout << "Kernel warning was unset." << std::endl;
+    }
+
+    void FlushErrorHandler(std::string_view error) override
+    {
+        std::cout << error << std::endl;
+    }
+
+    void FatalErrorHandler(std::string_view error) override
+    {
+        std::cout << error << std::endl;
+    }
+};
+
 void run_verify_test(
     const ScriptPubkey& spent_script_pubkey,
     const Transaction& spending_tx,
@@ -401,6 +430,7 @@ BOOST_AUTO_TEST_CASE(btck_context_tests)
         ChainParams regtest_params{ChainType::REGTEST};
         CheckHandle(params, regtest_params);
         options.SetChainParams(params);
+        options.SetNotifications(std::make_shared<TestKernelNotifications>());
         Context context{options};
     }
 }