Move `validate_enumerated_option`, `validate_positive_option`,
and `validate_factor_option` into shared index option utilities
under the `secondary_index::util` namespace. These functions were
previously defined as file-local statics in `vector_index.cc` with
hardcoded index names in error messages.
The shared versions take `index_type_name` as a parameter, allowing
each `custom_index` subclass to pass its own name via the virtual
`index_type_name()` method at the call site. The options maps use
`std::bind_front` to bind config params (supported values, limits),
leaving `index_name` as the first unbound argument passed by
`check_index_options()`.
Add `index_type_name()` as a pure virtual method on `custom_index`.
Move the shared utility implementations into `index_option_utils.cc`
and update `vector_index.cc` to use them.
