pgsql: Reject substituting extension schemas or owners matching ["$'\].

Reject substituting extension schemas or owners matching ["$'\].

Substituting such values in extension scripts facilitated SQL injection
when @extowner@, @extschema@, or @extschema:...@ appeared inside a
quoting construct (dollar quoting, '', or ""). No bundled extension was
vulnerable. Vulnerable uses do appear in a documentation example and in
non-bundled extensions. Hence, the attack prerequisite was an
administrator having installed files of a vulnerable, trusted,
non-bundled extension. Subject to that prerequisite, this enabled an
attacker having database-level CREATE privilege to execute arbitrary
code as the bootstrap superuser. By blocking this attack in the core
server, there's no need to modify individual extensions. Back-patch to
v11 (all supported versions).

Reported by Micah Gate, Valerie Woolard, Tim Carey-Smith, and Christoph
Berg.

Security: CVE-2023-39417

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d4648a74be07bfb23b449c722303c320297c0327

Modified Files
--------------
src/backend/commands/extension.c | 21 +++++++++++++++
src/test/modules/test_extensions/Makefile | 2 ++
.../test_extensions/expected/test_extensions.out | 30 ++++++++++++++--------
.../test_extensions/sql/test_extensions.sql | 17 +++++++++---
.../test_extensions/test_ext_extschema--1.0.sql | 5 ++++
.../test_extensions/test_ext_extschema.control | 3 +++
6 files changed, 63 insertions(+), 15 deletions(-)