[coro_http_client][feat]support out buf when request

[qicosmos]

Why

support out buffer when request http.

What is changing

Example

void use_out_buf() {
  using namespace coro_http;
  std::string str;
  str.resize(10);
  std::string url = "http://cn.bing.com";

  str.resize(6400);
  coro_http_client client;
  auto ret = client.async_request(url, http_method::GET, req_context<>{}, {},
                                  std::span<char>{str.data(), str.size()});
  auto result = async_simple::coro::syncAwait(ret);
  bool ok = result.status == 200 || result.status == 301;
  assert(ok);
  std::string_view sv(str.data(), result.resp_body.size());
  assert(result.resp_body == sv);
}