Use std::optional if available.

Currently, SDK requires C++11 minimum.
So, boost::optional type is used for optional values.
For C++17 and above more convenient is to use std::optional instead.
The task NLAM-23 is about making this type configurable.
This commit is a renaming of the initially proposed "optional.hpp"
into optional.h to avoid clang-format header sorting issue.

Relates-To: NLAM-23
Signed-off-by: sopov <[email protected]>

Author: asopov-here