MDEV-38144: update Optional_metadata_fields to use MariaDB types

[EricHayter]

Currently Optional_metadata_fields has many members that use classes from the C++ standard library, most notably the use of std::vector and std::string. These members have been updated to use existing MariaDB types instead.

[EricHayter]

Running .clangformat seemed to add lots of line changes. Working on providing a separate commit just with the main changes to make review easier.

[grooverdan]

Running .clangformat seemed to add lots of line changes. Working on providing a separate commit just with the main changes to make review easier.

I think it was determined that the coding standard wasn't acheivable with clang-format. I'm not sure if worth the effort.

[bnestere]

Hi @EricHayter !

Thank you for submitting the PR! It is good to update our types to be consistent with MariaDB types. One thing this PR misses, that is in the description of MDEV-38144, is that rather than having an array per field, it would be better to have one array that has elements of a complex type which describe a field.

Previously, another contributor had prepared a patch, PR #4494, which looked to satisfy this; however, it was never thoroughly reviewed because they never got it to compile / pass tests on our CI system. I'd suggest looking through it for some inspiration (though I'm not sure of the quality of it).

Something the previous patch didn't try (though the JIRA mentions) is to use the Column_definition type defined in sql/field.h. That would be another improvement.