Allow extensions correctly adding custom columns to phpBB users table

If an extension adds custom column to the native phpBB table like USERS_TABLE, but the type of the column doesn't allow assigning default value (which is the case for the TEXT/BLOB data types in MySQL f.e.), phpBB will throw error on users registration (and another actions if appropriate) while the ext is disabled:

SQL ERROR [ mysqli ]
Field 'collapsible_categories' doesn't have a default value

The idea is 1) Add a new DBAL method(s) to get columns default values, 2) Adjust user_add() procedure to get table columns which don't have default values and assign values to them, just like it's currently done for $additional_vars (just for example).