Add support for the COMMENT field in DBForge and MySQL Forge classes (pdo, mysql, and mysqli)

Signed-off-by: Zachary Flower <zach@ninjaninja.net>
Disable database COMMENT field by default in all other DB Forge drivers

Signed-off-by: Zachary Flower <zach@ninjaninja.net>
Ensure forge comments are actually getting parsed

Signed-off-by: Zachary Flower <zach@ninjaninja.net>
Move comment field after unique field in db forge
Move MySQL comments to MySQL forge classes only
Remove comment field definition from all other forge classes
Merge pull request #3305 from zachflower/feature/mysql-comments

Add COMMENT support to MySQL Forge Drivers
Polish changes following #3305

Also add support for field comments in PostgreSQL & Oracle ... only via alter_table() for now :/