Optional_metadata_fields Replace std::vector with MariaDB Types

The Table_map_log_event::Optional_metadata_fields struct is full of std::vector types, one for each column option/type/attribute etc. Instead, it would be better to have an array of fields. This would allow us to allocate everything in one go (except column names) and would simplify a lot of the current functions. Even better would be to use Column_definition to hold the values.

This work split from MDEV-37674, which covered part of the Optional_metadata_fields refactoring to remove uses of std::string.