kernel: Add progress method to notifications

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.

src/rpc/blockchain.cpp

@@ -1123,7 +1123,7 @@ static RPCHelpMan verifychain()
     LOCK(cs_main);
 
     Chainstate& active_chainstate = chainman.ActiveChainstate();
-    return CVerifyDB().VerifyDB(
+    return CVerifyDB(chainman.GetNotifications()).VerifyDB(
                active_chainstate, chainman.GetParams().GetConsensus(), active_chainstate.CoinsTip(), check_level, check_depth) == VerifyDBResult::SUCCESS;
 },
     };