Problem/Motivation
MySQL and MariaDb (sort of) supports JSON fields.
We don't have the type defined in \Drupal\Core\Database\Driver\mysql\Schema::getFieldTypeMap() which makes working with them trickier than it should be.
There's https://www.drupal.org/project/json_field but if you use that and then try to use the DbDumpCommand it breaks because we're missing information in the schema type map.
Proposed resolution
Add the information to the schema type map.
Remaining tasks
Add a test.
User interface changes
None
API changes
None
Data model changes
None