feat(pgbouncer): exporter can use separate auth secret

[masterkain]

Supersedes #30 (which is conflicting/outdated and not back-compatible).

What

• Add optional pgbouncerExporter.auth.existingSecret (+ userKey/passwordKey) for exporter credentials.
• Default behavior unchanged: exporter continues to use config.existingAdminSecret/generated admin secret when auth.existingSecret is empty.
• Add charts/pgbouncer/ci/install-values.yaml so ct install can run (provides a dummy admin password).
• Add a hard guard: fail template if both PDB fields are set (Kubernetes disallows both).
• Bump chart version: 4.1.0 -> 4.1.1.
• Fix Chart maintainer name for ct validation.

How To Use

• Create a Secret with keys matching userKey/passwordKey (defaults adminUser/adminPassword) and set pgbouncerExporter.auth.existingSecret.
• Also ensure PgBouncer allows that user for stats if needed (e.g. set config.pgbouncer.stats_users and add the user to config.userlist).

Validation (local)

• helm lint charts/pgbouncer
• helm template + kubeconform
• ct lint --target-branch main
• ct install --charts charts/pgbouncer against Docker Desktop Kubernetes