[8.x] Add MySQL column modifier to set current timestamp on update

[cerbero90]

This PR adds the column modifier useCurrentOnUpdate() to set the current timestamp to a column when a record is updated.

Usage

$table->timestamp('updated_at')->nullable()->useCurrentOnUpdate();

This functionality is only supported by MySQL whereas other DBMS require a trigger.

Optional feature
We may also think of updating the Blueprint::timestamps() method to set timestamps automatically, useful for example when we insert/update records via query builder and not through Eloquent:

public function timestamps($precision = 0)
{
    $this->timestamp('created_at', $precision)->useCurrent();

    $this->timestamp('updated_at', $precision)->nullable()->useCurrentOnUpdate();
}

Please let me know what are your thoughts and I'll add it to the PR if it may be useful in your opinion.

If the PR gets merged, I'll make sure to update the docs as well.