postgres_fdw: Fix deparsing of remote column names in stats import.

postgres_fdw: Fix deparsing of remote column names in stats import.

build_remattrmap() deparses a list of remote column names for a query
that retrieves attribute stats for them from the remote server.
Previously, it did so by using the array-literal syntax with each column
name individually quoted by quote_identifier(), causing the query to
fail on the remote server with a syntax error or no results when that
column name included a single quote or backslash, as quote_identifier()
doesn't escape those characters, making the query invalid or incorrect.
Fix by switching from the array-literal syntax to the ARRAY constructor
syntax with each column name individually quoted by
deparseStringLiteral().

Oversight in commit 28972b6fc.

Reported-by: Satya Narlapuram <satyanarlapuram@gmail.com>
Reported-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: Alex Guo <guo.alex.hengchen@gmail.com>
Reviewed-by: Zhenwei Shang <a934172442@gmail.com>
Reviewed-by: Etsuro Fujita <etsuro.fujita@gmail.com>
Discussion: https://postgr.es/m/CAHg%2BQDc9%3DWtYi%3DJW6QUL6ASOJc6PcGPTuxoMkhnkQ7oi7j5atg%40mail.gmail.com
Discussion: https://postgr.es/m/CAJTYsWWGhVDFjr%2BsmdYdU-Q_TT9YMzXA4QcLCr7rizDOyrEEow%40mail.gmail.com