feat: Allow use of table and column aliases for table functions returning unknown types

[haines]

At the moment, when a table function returns a type that isn't a known table, it's excluded from the output columns completely. This makes it impossible to use unnest() or unnest() WITH ORDINALITY with a table alias (#1205, #1596).

This PR allows constructs such as

SELECT foo.bar FROM unnest(baz) AS foo (bar);

and

SELECT foo.bar, foo.i::bigint FROM unnest(baz) WITH ORDINALITY AS foo (bar, i);

I haven't attempted to solve this problem in general - it will only work for a table alias with column alias(es), and data types are any, necessitating casts. This seems like a good first step, at least.

Fixes #1205

[StevenACoffman]

I really appreciate your continued work on this area, as you appear to be fixing our biggest pain points! This and your work on #1884 are much appreciated, and I hope both can get reviewed and merged soon!

[haines]

Thanks @StevenACoffman, I'm glad this is helpful!