Yii2: Add ability to use \yii\db\Expression for SchemaBuilder::defaultValue()

[alxkolm]

Add ability to use \yii\db\Expression for SchemaBuilder::defaultValue().
Particularly this need for assign default value to TIMESTAMP column:
$this->timestamp()->defaultValue(new \yii\db\Expression('CURRENT_TIMESTAMP'))

[klimov-paul]

Perhaps we should introduce another method defaultExpression() for expressions to simplify syntax:

$this->timestamp()->defaultExpression('CURRENT_TIMESTAMP')

[cebe]

:+1:

[yupe]

+1

[bajabob]

+1