rgw/curl: async curl client

[cbodley]

note that this PR is based on top of ceph#50064 which targets Ceph's wip-coro-after-reef feature branch

in contrast to RGWHTTPManager's background thread that polls on curl_multi_wait()/curl_multi_perform(), this uses a different flavor of the curl_multi API that integrates much better with boost::asio:

// This implementation uses the 'multi_socket' flavor of the libcurl multi API:
// https://curl.se/libcurl/c/libcurl-multi.html
//
// By providing our own CURLMOPT_TIMERFUNCTION AND CURLMOPT_SOCKETFUNCTION, we
// can do all necessary polling and waiting on the given asio executor. As we
// get wakeups from asio, we call curl_multi_socket_action() to do its
// non-blocking socket i/o from within that executor.
//
// This also overrides CURLOPT_OPENSOCKETFUNCTION on each request to manage the
// pool of asio sockets.

[cbodley]

cc @adamemerson @mkogan1 @yuvalif