Add `isready` initContainer for externalDatabases

Describe your Issue

https://github.com/nextcloud/helm/blob/3b7b2f24ca5c8da56b4ad29b80e66ded4a6da69a/charts/nextcloud/templates/deployment.yaml#L276
https://github.com/nextcloud/helm/blob/3b7b2f24ca5c8da56b4ad29b80e66ded4a6da69a/charts/nextcloud/templates/deployment.yaml#L299

      {{- if or .Values.nextcloud.extraInitContainers .Values.mariadb.enabled .Values.postgresql.enabled .Values.externalDatabase }}
      ....
      {{- if .or Values.mariadb.enabled (eq .Values.externalDatabase.type "mysql")  }}
      ....
      {{- else if or .Values.postgresql.enabled (eq .Values.externalDatabase.type "postgresql") }}

there is such mecanism for db from chart dependencies
but none form externalDB
would be great to add the condition for the external db

Logs and Errors

none

Describe your Environment

• Kubernetes distribution:OVH kubernetes

• Helm Version (or App that manages helm): helm

• Helm Chart Version: 4.5.10

• values.yaml:

  ingress:
    enabled: true
    ingressClassName: nginx
    hostname: "testwordpress.xxxxx.fr"
    path: /
    tls: true
    # secrets:
    #   - name: "testwordpress-secret"
  externalDatabase:
    host: mariadb.svc.cluster.local
    user: userwordpress
    password: xxxxxxx
    database: wordpress
    port: 3306
  mariadb:
    enabled: false
  wordpressUsername: admin
  wordpressPassword: dk435lej15466cCZE
  allowEmptyPassword: false
  persistent:
    selector:
      name: wordpress

Additional context, if any

None

Start in charts/nextcloud/templates/deployment.yaml around lines 276 and 299, where init containers are conditionally rendered for chart-managed databases. Render the chart with externalDatabase configured and the bundled databases disabled, then compare the output with the existing database cases. Done means the external database receives the intended isready initContainer without breaking other configurations.