- remove [<initializer_list>](https://en.cppreference.com/w/cpp/header/initializer_list) from internal header. It declares `std::initializer_list`, `std::begin` and `std::end`, none of which are used there
- add `absl/base/attributes.h` to the internal header because it provides `ABSL_FALLTHROUGH_INTENDED`
- add [<functional>](https://en.cppreference.com/w/cpp/header/functional) to the public header, because it provides `std::reference_wrapper`

PiperOrigin-RevId: 632018125
Change-Id: Icf7a9c8a920c9c076ef416ea80995e3a81ce3e81