Support COLLECT operator

User would like the ARRAY(SELECT ...) operator, which becomes COLLECT.

Simple example:
```

SELECT ARRAY(SELECT f_int FROM PCOLLECTION)

```

We also want to make sure the original (more complex) example works:
```

SELECT
  otherScalarField,
  ARRAY(SELECT row_field1 FROM UNNEST(array_of_nested_rows)
FROM
  table

```

Imported from Jira [BEAM-11872](https://issues.apache.org/jira/browse/BEAM-11872). Original Jira may contain additional context.
Reported by: apilloud.