Add support for generated columns to the DB Schema API

Problem/Motivation

#3343634: Add "json" as core data type to Schema and Database API provides the ability to use JSON columns. After that's done, we'll need the ability to add indexes for the data in those columns.

There are two ways to achieve this: indexes on SQL expressions or indexes on generated columns. Expression indexes were attempted in the issue mentioned above, but they're not supported by MariaDB, and it's not realistic to exclude such a large user base from this.

Therefore, the only remaining option is to add support for generated columns, which can then be part of indexes just like regular ones.

Proposed resolution

Allow creating generated columns in the DB Schema API.

Remaining tasks

TBD.

API changes

API additions TBD.

Data model changes

Nope.

Release notes snippet

TBD.