Add volumeMounts option for frontend init container

[jdpleiness]

Allow for volumeMounts in the frontend init container.

• Allows mounting TLS certificates needed to establish encrypted TLS connections to PostgreSQL databases.

Values can be set in a override file as follows:

frontend:
  extraVolumes:
  - name: migrator-tls
    secret:
      secretName: migrator-tls-secret

migrator:
  extraVolumeMounts:
  - name: migrator-tls
    mountPath: /etc/tls
    readOnly: true
  env:
    PGSSLCERT:
      value: /etc/tls/tls.crt
    PGSSLKEY:
      value: /etc/tls/tls.key

Checklist

• Follow the manual testing process
• Update changelog
• Update Kubernetes update doc

Test plan

Manual test and Helm lint

[michaellzc]

can we port this change to the sourcegraph-migrator chart as well?