[PATCH] Fix column name escaping in postgres_fdw stats import

Hi hackers,

The new statistics import feature in postgres_fdw (commit 28972b6fc3d)
builds a remote query to fetch pg_stats rows, filtering by column name
with:

  AND attname = ANY('{col1, col2}'::text[])

The column names are formatted with quote_identifier(), which only
escapes double quotes.  But since the list is embedded inside a
single-quoted string literal, any single quote in a column name
breaks the literal and produces a syntax error on the remote server.

Reproduction:

  CREATE TABLE t ("col'quote" int, c2 int);
  INSERT INTO t SELECT g, g FROM generate_series(1,100) g;
  ANALYZE t;

  CREATE FOREIGN TABLE ft ("col'quote" int, c2 int)
    SERVER loopback OPTIONS (table_name 't', restore_stats 'true');

  ANALYZE ft;
  -- ERROR:  syntax error at or near "quote"
  -- CONTEXT:  remote SQL command: ... attname = ANY('{...
"col'quote"}'::text[])

The attached patch switches to an ARRAY[] constructor with each
element escaped by deparseStringLiteral(), matching how schemaname
and tablename are already handled in the same function.

Thoughts?

It should also address the issue that was raised in [1].

[1] PostgreSQL: Fix array-element quoting in postgres_fdw import statistics
<https://www.postgresql.org/message-id/CAHg+QDc9=WtYi=JW6QUL6ASOJc6PcGPTuxoMkhnkQ7oi7j5atg@mail.gmail.com>

Regards,
Ayush