Add generic request function in api.h

[GitSparTV]

Provides generic function to call any HTTP method.

This helps to avoid such code on your side:

template<class ...Ts>
cpr::Response MakeRequest(std::string_view method, Ts&& ...ts) { // could be enum, string, or bool. Whatever what you use to select different HTTP method
    if (method == "GET") {
        return cpr::Get(std::forward<Ts>(ts)..., cpr::Header{{"Authorization", "token"}});
    else if (method == "POST") {
        return cpr::Post(std::forward<Ts>(ts)..., cpr::Header{{"Authorization", "token"}});
    } else {
        // and so on...
    }
}

Now you can do this:

    template <cpr::HTTPMethod method, typename... Args>
    cpr::Response MakeRequest(Args&&... args) {
        return cpr::Request<method>(std::forward<Args>(args)..., cpr::Header{{"Authorization", "token"}});
    }

and call it like this:

cpr::Response r = MakeRequest<cpr::HTTPMethod::kPost>(cpr::Url{"http://www.httpbin.org/post?a=b"}, cpr::Body{"{\"a\": true}"});

Ideas before the merge:

• Do we need tests for this function?
• This function can replace all existing functions like Post(), Get(), and other! Should I refactor it?
• HTTPMethod doesn't have Download since it's not a method. Do we need it?
• Do we need Async and/or Multi versions?

Resolves #1117

[COM8]

@GitSparTV Tanks for this PR! Looks good.
Could you please add a few test (examples) here: https://github.com/libcpr/cpr/tree/master/test

And a bit of documentation here: https://github.com/libcpr/docs

[COM8]

@GitSparTV I'm closing this one for now since things are missing. Feel free to reopen in case you want to finish this PR.