Switch to std::optional instead of Azure::Nullable

[RickWinter]

There are some open source implementations that would enable optional without the need to be C++17.

This is an interesting read.  

Functional exception-less error handling with C++23's optional and expected - C++ Team Blog (microsoft.com)

TartanLlama/optional: C++11/14/17 std::optional with functional-style extensions and reference support (github.com)

C++11/14/17 std::optional with functional-style extensions and reference support - GitHub - TartanLlama/optional: C++11/14/17 std::optional with functional-style extensions and reference support