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/validation.h

@@ -955,6 +955,7 @@ public:
     bool ShouldCheckBlockIndex() const { return *Assert(m_options.check_block_index); }
     const arith_uint256& MinimumChainWork() const { return *Assert(m_options.minimum_chain_work); }
     const uint256& AssumedValidBlock() const { return *Assert(m_options.assumed_valid_block); }
+    kernel::Notifications& GetNotifications() const { return m_options.notifications; };
 
     /**
      * Alias for ::cs_main.