Enable more flexible Datalab storage configuration

[achtsnits]

Currently, each team receives the following storage resources:

• File Storage:
  A default 1 GB persistent volume using the cluster’s default storage class.

• Object Storage:
  One or more S3-compatible buckets (self-service possible), accessible via credentials and also browsable through the File Browser (mounted using a CSI driver with FUSE).

Teams and Applications running on top of a datalab may have different prerequisites.

Goals

• Make the file storage maximum quota configurable.
• Make the file storage class name configurable.
• Allow to adapt PSP and Kubernetes access

Future Evolution (to be tracked separately, not here)

• CSI FUSE related - Datalab storage configuration in regards to CSI-FUSE #86
• Support for additional storage types, e.g. relational databases.
• Enforcing object storage quotas.
• Introducing individual per-team storage quotas.

[achtsnits]

new configurable security and quota options got introduced -> part of provider-datalab 0.2.0 release, can be set per DataLab or globally via EnvironmentConfig

README and documentation got updated, Usage & Concepts now include detailed examples with inline comments, Security introduces spec.security parameters (policy, kubernetesAccess, kubernetesRole) and explains how they interact with Pod Security Standards (restricted, baseline, privileged) to define runtime permissions and access levels