Add support for CERN PGBouncer chart to GitLab chart

Summary

Add CERN's PGBouncer Chart to GitLab sub-charts.

Concerns directly for GitLab's integration:

• Maintenance of the image & chart: Who will be responsible?
• We need to be able have solid documentation, and some troubleshooting items available, so that Support can help customers that run into issues.
• We need to know distinct value in excess of the current implementation via external PgBouncer (see Reference Architectures), in order to consider this a good replacement option.