Use std::iterator_interface from P2727 for Beman.Optional26

[neatudarius]

As discussed in a previous Beman sync meeting (06-17-2024), as a long-term approach, we want to use std::iterator_interface from https://wg21.link/P2727R4 into the Beman project, and thus, in Beman.Optional26.

Note: Check Boost implementation.

• [x] Initial implementation with copied sources from boostorg/stl_interfaces. Author provided permission here.
• [ ] Actually integrate stl_interfaces into Beman project. TBD.

[neatudarius]

18 used a copy of few headers from https://github.com/boostorg/stl_interfaces to fully implement P3168R1.

TBD if stl_interfaces will be implemented in Beman Project or what long-term approach do we pick.

CC: @tzlaine @steve-downey @camio