Support custom `ORDER BY` for virtual fields added via `AddField`

In my project, I use info.AddField to add virtual (computed) columns to the list page that are not present in the database table — for example:

info.AddField("Groupe", "display_count", db.Varchar).
    FieldDisplay(func(model types.FieldModel) interface{} {
        cnt, _ := db.WithDriverAndConnection("default", condb).
            Table("map_message_groups").
            Where("message_id", "=", model.Row["uid"]).Count()
        return cnt
    }).
    FieldSortable()

The problem is:

• FieldSortable() works only for real DB columns.
• Virtual columns computed in FieldDisplay cannot currently be sorted, because there is no FieldOrderBy() (or similar) API for AddField to define a custom SQL expression for ordering.

I want to keep these virtual columns without altering the database schema, but still allow server‑side sorting via a correlated subquery or custom SQL expression.

Impact:

• Common for dashboards to show counts or derived data without adding DB columns.
• Without this capability, sorting requires either:
  • Fetching all rows and sorting in memory (slow, breaks pagination), or
  • Creating extra DB columns just for ordering (undesirable for many projects).

Suggested enhancement:

• Add an API similar to ColumnOrderBy for AddField, e.g.:

.FieldOrderBy(func(order string, table string) string {
    return fmt.Sprintf("(SELECT COUNT(1) FROM map_message_groups mg WHERE mg.message_id = %s.uid) %s", table, order)
})

This would let developers provide a custom ORDER BY expression for non‑DB fields while keeping current pagination and efficiency.