Add support for S3 storage

[Martin Gruner]

Zammad 6.2 was released with support for S3 as a file storage backend. This is probably the best to use in k8s environments.

We want to add support for it in the helm chart:

•  Add https://artifacthub.io/packages/helm/bitnami/minio as a new dependency chart
• Allow it to be configured or turned off
• Allow usage of external S3 services
• Configure Zammad to use it by default for new installations
• Add configuration instructions
• Add instructions on how to migrate existing FS storage content to S3

We can probably release this as a new minor version. It prepares a migration path for #243.