aggregate entities

currently `arrayAgg` from `Database.Esqueleto.PostgreSQL` allows to aggregate `Value`s.

It would be nice if it was possible also to aggregate entities.

For example, consider a 1:n relation between tables `foo` and `bar`. IT would be nice to write

```
data Baz = Baz
{ bazFoo :: Entity Foo
, bazBars :: [Entity Bar]
}

$(deriveEsqueletoRecord ''Baz)

query :: FooId -> SqlPersistT m [Baz]
query fooId =
select $ do
foo :& bar <-
from $
table @Foo
`leftJoin` table @Bar
`on` (\(foo :& bar) -> just foo.id ==. bar.foo)
where_ $ foo.id = val fooId
groupBy foo.id
pure $
SqlBaz
{ bazFoo = foo
, bazBars = arrayAgg bar
}
```
or something analogous