Auto-enable pg_trm extension for postgres during DB creation.

[weitzman]

No description provided.

[phil-s]

This should also happen in dropOrCreate().

This is to cover the scenario where the database is provisioned by some external process which doesn't know about Drupal-specific requirements and therefore doesn't create the pg_trgm extension. In that situation, drush site:install calls dropOrCreate() which drops all the tables but does not create the extension, and consequently the process errors out a bit later.

As CREATE EXTENSION IF NOT EXISTS pg_trgm is a no-op for the cases where the extension did already exist, I don't see a reason not to do this.

[phil-s]

And although it looks to me as if nothing would normally call SqlPgsql::createdbSql(), I think the command should be included there as well.