feat: allow secretRef for admin password, token, and s3 credentials

[mladedav]

Right now we have to either have the secrets in plaintext in values.yaml or we have to use `extraEnvVars workaround like this:

extraEnvVars:
  - name: KELLNR_S3__SECRET_KEY
    valueFrom:
      secretKeyRef:
        ...

This allows to set the secretRef for admin password and token and for s3 credentials in the same way as can be done for oauth2 secret and cookie signing secret.

[secana]

Thanks! Very welcome addition. Two small things before merge:

1. Bump the chart version in Chart.yaml (6.0.1 → 6.1.0) — that's what's making CI red (ct lint requires a version bump), and chart-releaser won't publish the feature without it.
2. Add README docs for the four new refs (adminPwdSecretRef, adminTokenSecretRef, S3 accessKeySecretRef/secretKeySecretRef), matching the table we have for cookieSigningKeySecretRef.

[mladedav]

I've bumped the version and added the docs.

There was no section for s3 though so I've created one. I didn't fill it with all options, just the new ones.