ci: verify Helm Chart with snapshot tests

[apricote]

TL;DR

Verify the functionality of the Helm Chart by adding snapshots for some example value files. This showcases changes to the rendered manifests.

Expected behavior

Same as in csi-driver:

• Committed snapshots: https://github.com/hetznercloud/csi-driver/tree/main/chart/.snapshots
• Script to update snapshots: https://github.com/hetznercloud/csi-driver/blob/main/hack/update-helm-snapshots.sh
• Tests for up-to-date snapshots in CI: https://github.com/hetznercloud/csi-driver/blob/4c77198f9138dfc672cedea184551de2c20857b0/.github/workflows/test.yml#L132-L155

We can start with the values.yaml we have right now and possibly add one similar to the example-prod in csi-driver.