make qtoken for PostgreSQL _fk_regex_pattern less restrictive

make qtoken for PostgreSQL _fk_regex_pattern less restrictive

Improved the foreign key reflection regular expression pattern used by the
PostgreSQL dialect to be more permissive in matching identifier characters,
allowing it to correctly handle unicode characters in table and column
names. This change improves compatibility with PostgreSQL variants such as
CockroachDB that may use different quoting patterns in combination with
unicode characters in their identifiers.  Pull request courtesy Gord
Thompson.

Change-Id: Iaee340879400e01df2f776417e8b1018f1801cfe
(cherry picked from commit d0d9f1b71115471b0a6918075383c2bddf2212b1)