pgsql mix-and-match of default/defined connection

[ZNeumann]

The current pgsql agent code (and test) does not allow mixing of the default connection with a defined connection when preparing and executing a request. The test that exercises this functionality is marked xfail.

Relevant files:

• agent/php_internal_instrument.c
• tests/integration/pgsql/test_pg_execute_mixed_use.php

This feature (and use of the default connection in general) is deprecated in php 8.1