Reference Clickhouse credentials in secret

[namm2]

Use secretKeyRef to not expose Clickhouse credentials into runtime environment

[CLAassistant]

All committers have signed the CLA.

[HamseTelia]

Could we also please add the clickhouse user's password to be referenced from a secret clickhouse.config.users.appUserPassword. Like suggested here: #125

clickhouse:
  config:
    users:
      # Existing inline password support (for backward compatibility)
      appUserPassword: "hyperdx"
      otelUserPassword: "otelcollectorpass"
      otelUserName: "otelcollector"
      
      # NEW: External secret support
      useExternalSecret: false
      externalSecret:
        name: ""
        appUserPasswordKey: "app-user-password"