Make per-index options modifiable via instant DDL

MDEV-37070 introduces per-table and per-index options.
Changing the added per-table option adaptive_hash_index is possible via instant DDL (see alter_options_need_rebuild()).
The same does not hold for per-index options (see compare_keys_but_name(), Compare_keys, ha_innobase::check_if_supported_inplace_alter()).
There may be per-index options that genuinely require index rebuild, but for those that do not (such as adaptive_hash_index, complete_fields, bytes_from_incomplete_field, for_equal_point_to_last_record, and the upcoming shrink from MDEV-39851), this feature would allow instant change of the option.