Refactor AsyncWrapper to make it safer

[Anohkka]

It's currently possible to construct an unsafe cancelable AsyncWrapper by explicitly specifying the template arguments in the constructor, like so:

#include "cpr/cpr.h"

int main(int, char**) {
    auto wrapper = cpr::AsyncWrapper<void, true>{std::future<void>{}};
    
    // Dereferences a null pointer
    auto _ = wrapper.IsCancelled();
}

To prevent that, we split AsyncWrapper it into two template specializations. AsyncWrapper<T, false> keeps most of the code, while cancelation-related code is made exclusive to AsyncWrapper<T, true>. Most importantly, we can prevent a cancelable AsyncWrapper from being constructed without a provided pointer. AsyncWrappers that aren't cancelable get to be a bit slimmer, no longer containing the pointer to keep track of cancelation state.