Preserve the name of column after unnest

[oneonestar]

Motivation

Before #10883, we were able to do the following selection:
SELECT a.field1, a.field2 FROM UNNEST(...) t(a)

After the patch, this kind of query become impossible. The name of the fields becomes _col0, _col1. Column aliases is required in order to select the needed columns. We could rewrite it in this way:
SELECT field1, field2 FROM UNNEST(...) t(field1, field2)

However, it is infeasible to use column aliases to refer to a row with a lot of fields. I think we should preserve the name of the columns so that we can refer to the column easily, as below:

# Hard to use column aliases for complicated row (after #10883):
SELECT field1, field100 FROM UNNEST(...) t(field1, field2, field3, ..., field100)

# Preserve the name of the columns will allow this:
SELECT field1, field100 FROM UNNEST(...) t

---

What changes were proposed in this pull request

Before:

presto> SELECT * FROM UNNEST(array[
     ->     CAST(ROW('a', 1) AS ROW(x varchar, y int)),
     ->     CAST(ROW('a', 1) AS ROW(x varchar, y int))
     -> ]);
 _col0 | _col1
-------+-------
 a     |     1
 a     |     1
(2 rows)

After:

presto> SELECT * FROM UNNEST(array[
     ->     CAST(ROW('a', 1) AS ROW(x varchar, y int)),
     ->     CAST(ROW('a', 1) AS ROW(x varchar, y int))
     -> ]);
 x | y
---+---
 a | 1
 a | 1
(2 rows)

[martint]

Indeed, this is a bug in #10883. According to the SQL specification:

ii) Otherwise, Case:
  1) If any ETj, 1 (one) ≤ j ≤ NCV, is a row type, then:
    A) For each ETj that is a row type:
      I) Let DET(j) be the degree of ETj.
>>>>  II) Let FNj,i, 1 (one) ≤ i ≤ DET(j), be the name of the i-th field in ETj. 

[martint]

Overall, looks good, but please adjust the test cases based on the comments.

[martint]

It's not immediately obvious that array(cast(...) as row(x int, y varchar), cast(...) as row (x int, y varchar))) is of type
array(row(x int, y varchar)).

A better form would be: cast(array[row(1, 'a'), row(2, 'b')] as array(row(x int, y varchar)))

[martint]

Ok, I made the changes I suggested above and merged it. Thanks for you contribution!