Ensure PVCs can be cleaned up on cluster deletion

[Gerrit91]

Customers are reporting that currently some volumes are cleaned up on cluster deletion but some are left, which could be a race condition that occurs in the shoot deletion flow. We should investigated the behavior and try to establish a clean deletion flow for this controller.

References #28.

[Gerrit91]

Some ideas to solve this problem:

• Deploy lb-csi-controller stateful set into seed's shoot namespace (this way it cannot be deleted during the shoot cleanup)
• Build an extension for lightbits and not deploy it through GEPM, such that the cleanup phase is called before KubeAPIServer cleanup