Fix Extra Column import since MySQL 8

[twister65]

Pull Request for Issue #joomla/joomla-cms#37662

Summary of Changes

MySql 8.0 introduces DEFAULT_GENERATED in the extra column and should be replaced with the default value.

`Extra

Any additional information that is available about a given column. The value is nonempty in these cases:

auto_increment for columns that have the AUTO_INCREMENT attribute.

on update CURRENT_TIMESTAMP for TIMESTAMP or DATETIME columns that have the ON UPDATE CURRENT_TIMESTAMP attribute.

VIRTUAL GENERATED or STORED GENERATED for generated columns.

DEFAULT_GENERATED for columns that have an expression default value.`

See https://dev.mysql.com/doc/refman/8.0/en/show-columns.html

Testing Instructions

• Export all tables and data to the folder:

php cli/joomla.php database:export --folder=<folder_path>

• Import all from folder:

php cli/joomla.php database:import --folder=<folder_path>

Documentation Changes Required

[richard67]

See CMS issue joomla/joomla-cms#37662 .

[alikon]

i've tested successfully on mysql 8.0.28 (PDO)

[Llewellynvdm]

Okay @nibra or @HLeithner what is holding this fix back? Its been tested and it seems straightforward. I realize it has the dreaded if..else :) but as an initial step to fix this limitation going to mysql 8.0 it seems simple enough.

[nibra]

It's ok for me. Can't tell, why it was overlooked.

[HLeithner]

Nothing form my side.

[Llewellynvdm]

I re-based this PR, and merged it into 2.0-dev