Support custom expressions for a field

[Tobe Osakwe]

Example:

@serializable
@orm
class _Forum extends Model {
  @Column(expression: 'COUNT(SELECT * FROM posts WHERE forum_id=id)')
  int postCount;
}

[Tobe Osakwe]

Work begins: 860e802

[Tobe Osakwe]

Closing, as this is available in the latest beta.