Refactor boost::optional to std::optional after code base is migrated to C++17

`boost::optional` and `std::optional` have very similar interfaces, so it should be easy to change the naming. due to licensing issues we should use `std::optional` instead of `boost::optional` in the end.

AC:

- change all references of `boost::optional` to `std::optional`.
- remove references to ``

Search the repository for `boost::optional` and `` after confirming the code base is migrated to C++17. Replace every reference with the standard equivalent, remove the Boost optional includes, and verify that the project still builds and tests pass.

Written by the indexing model from the issue text.