Use the Drupal core pgsql module to do most of the database interactions

Problem/Motivation

The module currently makes use of direct calls to the PostgreSQL extension. Drupal provides PostgreSQL PDO bindings through the pgsql module.

Using a PDO rather than direct raw queries like the search_api_ai_pgvector module does seems like a much better way to handle it from a maintainance and security point of view.

Proposed resolution

Leverage Drupal's pgsql module to interact with the postgresql database.