Introduce RAII-Scope Guard instead of using `std::shared_ptr`

[Curve]

The http client currently uses an empty std::shared_ptr<int> as a scope-guard to run cleanup code:

yalantinglibs/include/ylt/standalone/cinatra/coro_http_client.hpp

Lines 1464 to 1468 in eec1eda

	std::shared_ptr<int> guard(nullptr, [this](auto) {
	if (!req_headers_.empty()) {
	req_headers_.clear();
	}
	});

If I'm not mistaken, this will still have to allocate a shared state to store the user defined destructor in, which is quite wasteful. I'd suggest to introduce a simple scope guard for this purpose instead of abusing std::shared_ptr for this:

template <typename T>
struct scope_guard
{
  T callback;

public:
  explicit scope_guard(T callback) : callback(std::move(callback)) {} 

public: 
  scope_guard(const scope_guard&) = delete;
  scope_guard(scope_guard&&) noexcept = delete;

public:
  ~scope_guard() { callback(); }
};

You could also leave out the deleted ctors and make it an aggregate for cleaner construction :)

[qicosmos]

Yes, but using std::shared_ptr as a guard is simpler. It is fine to improve it if you prefer.