pgsql: oauth: Recheck oauth_validator_libraries during authentication

oauth: Recheck oauth_validator_libraries during authentication

The validator allowlist is only enforced while parsing pg_hba.conf.
If changing oauth_validator_libraries makes an existing oauth line
invalid, the HBA reload fails and the old lines stay in use, so a
removed validator keeps authenticating users. In particular, when
setting the list to the empty string, the server doesn't refuse an
OAuth connection, as documented.

Backpatch down to v18 where OAuth was introduced.

Author: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: shihao zhong <zhong950419(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAN4CZFNbWXwC=s4t9Xu1kitLsQvTe-EwpVtFCjixWCHBxRTraQ@mail.gmail.com
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/8e54a414570010afed0b8b1ed10882afba380166

Modified Files
--------------
src/backend/libpq/auth-oauth.c | 14 ++++++++++++++
src/test/modules/oauth_validator/t/001_server.pl | 16 ++++++++++++++++
2 files changed, 30 insertions(+)