Support extraContainers and extraVolumes for cloud sql proxy sidecars

Continued from the discuss in #48 (closed).

For external db on GCP, it is common to use cloud sql proxy as sidecar. This can be done by providing extraContainers, extraVolumes and extraVolumeMounts options for deployments/jobs that need to talk to postgres. See keycloak chart for example configuration.

Since there are multiple subcharts talking to postgres, what'd be the best way to propagate these options without redefining sidecar for every deployment/job?