connection::async_exec() requires a copyable CompletionToken

[cbodley]

with gcc 13.1.1, boost 1.82, and redis develop branch at commit 7d16259:

#include <boost/asio.hpp>
#include <boost/redis.hpp>

struct move_only_handler {
  move_only_handler(const move_only_handler&) = delete;
  move_only_handler(move_only_handler&&) = default;

  void operator()(boost::system::error_code, std::size_t);
};

int main(int argc, char** argv)
{
  boost::asio::io_context ioctx;
  boost::redis::connection conn{ioctx};

  boost::redis::request request;
  boost::redis::response<std::string> response;
  conn.async_exec(request, response, move_only_handler{});
}

compilation fails with:

~/test/redis_exec $ g++ main.cc -Iboost/include -I redis/include
In file included from redis/include/boost/redis.hpp:12,
                 from main.cc:2:
redis/include/boost/redis/connection.hpp: In instantiation of ‘auto boost::redis::basic_connection<Executor>::async_exec(const boost::redis::request&, Response&, CompletionToken) [with Response = std::tuple<boost::system::result<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, boost::redis::adapter::error> >; CompletionToken = move_only_handler; Executor = boost::asio::any_io_executor]’:
redis/include/boost/redis/connection.hpp:354:30:   required from ‘auto boost::redis::connection::async_exec(const boost::redis::request&, Response&, CompletionToken) [with Response = std::tuple<boost::system::result<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, boost::redis::adapter::error> >; CompletionToken = move_only_handler]’
main.cc:18:18:   required from here
redis/include/boost/redis/connection.hpp:232:30: error: use of deleted function ‘move_only_handler::move_only_handler(const move_only_handler&)’
  232 |       return impl_.async_exec(req, resp, token);
      |              ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
main.cc:5:3: note: declared here
    5 |   move_only_handler(const move_only_handler&) = delete;
      |   ^~~~~~~~~~~~~~~~~
In file included from redis/include/boost/redis/connection.hpp:10:
redis/include/boost/redis/detail/connection_base.hpp:556:72: note:   initializing argument 3 of ‘auto boost::redis::detail::connection_base<Executor>::async_exec(const boost::redis::request&, Response&, CompletionToken) [with Response = std::tuple<boost::system::result<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, boost::redis::adapter::error> >; CompletionToken = move_only_handler; Executor = boost::asio::any_io_executor]’
  556 |    auto async_exec(request const& req, Response& resp, CompletionToken token)
      |                                                        ~~~~~~~~~~~~~~~~^~~~~