db/config: forward-declare boost options_description_easy_init

Reduces large dependency pull from boost.

Closes scylladb/scylladb#22748

db/config.hh

@@ -9,7 +9,6 @@
 
 #pragma once
 
-#include <boost/program_options/options_description.hpp>
 #include <unordered_map>
 
 #include <seastar/core/sstring.hh>
@@ -28,6 +27,12 @@
 #include "utils/dict_trainer.hh"
 #include "utils/advanced_rpc_compressor.hh"
 
+namespace boost::program_options {
+
+class options_description_easy_init;
+
+}
+
 namespace seastar {
 class file;
 struct logging_settings;