Support returning row sets from functions

There are some functions (like [`unnest`](https://www.postgresql.org/docs/15/functions-array.html#:~:text=%E2%86%92%20%7B1%2C2%2C3%2C4%7D-,unnest,-(%20anyarray%20)%20%E2%86%92)) that should return a set of elements. We currently do not support returning sets of elements with our current function framework. For example, the [`unnest` function](https://github.com/dolthub/doltgresql/blob/main/server/functions/unnest.go) should return a set of `AnyElement`

```go
// unnest represents the PostgreSQL function of the same name, taking the same parameters.
var unnest = framework.Function1{
Name: "unnest",
Return: pgtypes.AnyElement, // TODO: Should return setof AnyElement
Parameters: [1]pgtypes.DoltgresType{pgtypes.AnyArray},
Strict: true,
Callable: func(ctx *sql.Context, _ [2]pgtypes.DoltgresType, val1 any) (any, error) {
valArr := val1.([]interface{})
if len(valArr) == 0 {
return nil, nil
}
if len(valArr) == 1 {
return valArr[0], nil
}
return nil, fmt.Errorf("unnest() only supports single-element arrays")
},
}
```

There are skipped tests for `unnest` [here](https://github.com/dolthub/doltgresql/blob/main/testing/go/functions_test.go#L888)