[FEATURE]: Simulate Virtual Generated Columns for Postgres

### Feature hasn't been suggested before.

- [x] I have verified this feature I'm about to request hasn't been suggested before.

### Describe the enhancement you want to request

Possibly a niche, nice-to-have feature, but worth considering.

The lack of Virtual generated table columns in Postgres is quite a frustration, especially coming from a database engine like MSSQL. Our use cases do not lend themselves to stored generated columns due to use of `CURRENT_TIMESTAMP` and other mutable functions; this and the [other limitations](https://www.postgresql.org/docs/current/ddl-generated-columns.html) of stored generated columns reduce their usefulness to some degree.

Hence, the proposal is to add a `generatedVirtualAs` function to the column generation that allows SQL to be generated during a SELECT statement. For example:

```ts
export const myTable = pgTable("MyTable", {
// other columns
dateCompleted: datetime().notNull(),
virtualColumn: boolean().generatedVirtualAs(sql`"dateCompleted" < CURRENT_TIMESTAMP`)
});
```

Which would generate the following SQL during a SELECT query:

```sql

select "MyTable"."dateCompleted","MyTable"."dateCompleted" < CURRENT_TIMESTAMP as "virtualColumn" FROM "MyTable"
```

Thus, the virtual column is not actually created in the postgres table, but is rather simulated using an inline SQL statement. Full type-safety is preserved, such that `$inferSelect` contains `boolean` for `virtualColumn`, and `$inferInsert` omits it.

This could be a very powerful feature, considering that mutable functions, subqueries and references to other generated columns could be used, which are the main limitations of stored generated columns.