sql: implement col_description()

[jordanlewis]

This postgres builtin retrieves the comment, if present, on a column in
a table. We don't support commenting on columns, so for us this function
currently always returns NULL.

Resolves #12783.
The test query in that issue hasn't yet been added to the ORM test file because it also depends on pg_catalog.pg_collation, which is tracked in #12976.

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This change is 

[cuongdo]

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Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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pkg/sql/parser/builtins.go, line 1586 at r1 (raw file):

			Types:      NamedArgTypes{{"table_oid", TypeInt}, {"column_number", TypeInt}},
			ReturnType: TypeString,
			fn: func(ctx *EvalContext, args DTuple) (Datum, error) {

nit: replace ctx and args with _

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Comments from Reviewable

[jordanlewis]

Review status: 1 of 2 files reviewed at latest revision, 1 unresolved discussion.

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pkg/sql/parser/builtins.go, line 1586 at r1 (raw file):

Previously, cuongdo (Cuong Do) wrote…

nit: replace ctx and args with _

Done.

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Comments from Reviewable

[knz]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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Comments from Reviewable