Scope: Reintroduce support of CXX17 standard library as a polyfill option.

Motivation:

CXX-2625 removed logic to support CXX17 native polyfill. Example of removed code:

#elif defined(BSONCXX_POLY_USE_STD)

#include <string_view>

namespace bsoncxx {
namespace v_noabi {
namespace stdx {

using ::std::basic_string_view;
using ::std::string_view;

}  // namespace stdx
}  // namespace v_noabi

This ticket proposes adding back the option to use CXX17 native polyfill. This may enable easy removal of the bsoncxx implementation of the polyfill once C++17 is the minimum supported standard.

A CMake option may not be needed to select between the CXX17 polyfill and the bsoncxx polyfill. If CXX17 is the chosen standard, the CXX17 polyfill may be chosen unconditionally.