In the latest versions of PostgreSQL, using `pg_dump` on a PgBouncer
connection can cause a full GitLab outage. This happens because
`pg_dump` clears the search path and explicitly sets the schema for
every SQL query it runs. When PgBouncer is used in transaction pooling
mode, these connection settings persist and cause queries made by Rails
to fail since the `public` schema is not searched.

Currently there is no way to tell whether a connection is using
PgBouncer, so there is no way to prevent `pg_dump` from running.

To avoid causing an outage, we provide admins with a way to override the
database settings for the backup and restore task via environment
variables:

* `GITLAB_BACKUP_PGHOST`
* `GITLAB_BACKUP_PGUSER`
* `GITLAB_BACKUP_PGPORT`
* `GITLAB_BACKUP_PGPASSWORD`
* `GITLAB_BACKUP_PGSSLMODE`
* `GITLAB_BACKUP_PGSSLKEY`
* `GITLAB_BACKUP_PGSSLCERT`
* `GITLAB_BACKUP_PGSSLROOTCERT`
* `GITLAB_BACKUP_PGSSLCRL`
* `GITLAB_BACKUP_PGSSLCOMPRESSION`

Relates to:

* https://gitlab.com/gitlab-org/gitlab/-/issues/23211
* https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/3470