Set and propagate database connection limits

What does this MR do?

• Set connection limits to prevent DB resource exhaustion.
  • This aligns the configuration of the OpenBao chart with the one of the Runway deployment. See https://gitlab.com/gitlab-org/govern/secrets-management/gitlab-secrets-manager-container/-/merge_requests/24
  • This was surfaced in gitlab-org/gitlab#568356 (comment 2958079982). Query latency was higher than expected.
• Propagate maxIdleConnections and maxParallel.
  • Prior to this change, these chart params had no effect on OpenBao config.
  • However, maxConnectRetries was properly handled.
• Add unit tests for when maxIdleConnections, maxParallel, or maxConnectRetries is set.

See https://openbao.org/docs/configuration/storage/postgresql/

Author's Checklist

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Ensure a release milestone is set.
• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline on GitLab.com.
• When ready for review, MR is labeled workflowready for review per the MR workflow.

Expected

• Test plan indicating conditions for success has been posted and passes.
• Documentation is created or updated. Params not covered by https://docs.gitlab.com/charts/charts/openbao/.
• Tests are added.

Related issues

Surfaced in gitlab-org/gitlab#568356 (closed)