feat: Implement list_columns for datasources.

[vrongmeal]

Lists the columns information for datasources:

> CREATE EXTERNAL DATABASE "PgDb"
::: FROM postgres OPTIONS (
:::   host = 'pg.demo.glaredb.com',
:::   port = '5432',
:::   user = 'demo',
:::   password = 'demo',
:::   database = 'postgres',
::: );
Database created
>
> select * from list_columns("PgDb", public, nation);
┌─────────────┬───────────┬──────────┐
│ column_name │ data_type │ nullable │
│ ──          │ ──        │ ──       │
│ Utf8        │ Utf8      │ Boolean  │
╞═════════════╪═══════════╪══════════╡
│ n_nationkey │ Int32     │ true     │
│ n_name      │ Utf8      │ true     │
│ n_regionkey │ Int64     │ true     │
│ n_comment   │ Utf8      │ true     │
└─────────────┴───────────┴──────────┘

Syntax:

-- All the three arguments are identifiers so they can either be double
-- quoted or quoteless.
--
-- Prefer adding `"` (double quotation marks) on cloud.
--
select * from list_columns("<database>", "<schema>", "<table>");

Fixes #1540

[vrongmeal]

I have opened a draft PR. Seems easy to implement, will do it for all other data sources and get it merged.

[scsmithr]

Ship!

[greyscaled]

Exciting times