[smart_holder] Implement try_as_void_ptr_capsule as a free function (pybind#4539) (#30013)

* Move try_as_void_ptr_capsule out from modified_type_caster_generic_load_impl.

* Try fixing clangtidy

Co-authored-by: Xiaofei Wang <6218006+wangxf123456@users.noreply.github.com>

Author: rwgk