Template CxPlatAsync

[nibanks]

Updates CxPlatAsync interface so that the input type is templated and removes the return type so that callers can remove all the extra casting as well as we provide compile-time type safety. The downside is that you must explicitly specify the template types (though you can always define your own typedef). Example usages (from test code):

CxPlatAsync Async([](void*) -> void {
    // no-op
});

uint32_t Ctx = 0;
CxPlatAsyncT<uint32_t> Async([](uint32_t* Ctx) -> void {
    *Ctx = 123;
}, &Ctx);