[Feature] Add clock skew computation and unit test

[kai-ion]

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Description of changes:
Add clock skew computation and unit test

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[sbiscigl]

ok im sort of split on this, nice to have wont block it, the reason you are doing all of this is because std::chrono::abs is in c++17. i sort of hate we are forced to do this the wrong way. so a suggestion. in aws/core/utils/local you could add Chrono.h and have something like

namespace Aws {
#if defined(__cpp_lib_chrono) && __cpp_lib_chrono >= 201510L
using std::chrono::abs;
using std::chrono::ceil;
using std::chrono::floor;
using std::chrono::round;
#else
// TODO: delete this branch once the minimum standard is C++17.
template <class Rep, class Period>
constexpr std::enable_if_t<std::numeric_limits<Rep>::is_signed,
                           std::chrono::duration<Rep, Period>>
abs(std::chrono::duration<Rep, Period> d) {
    return d >= d.zero() ? d : -d;
}
#endif
}

then here you could just call Aws::chrono::abs, and if you compile with 17 you get the feature, if you compile without you get the shim implementation.