test: Test snapshots in S3

[siegfriedweber]

Description

• Add an integration test to test snapshots in S3
• Use the ListenerClass external-unstable because the listener-operator now assumes ephemeral-nodes by default (see https://github.com/stackabletech/listener-operator/pull/340/files#diff-e075573c368272b4008359c7176d058add8ef511a4dfeefdfddbeddbc92a93bcR141). This means that the ListenerClass external-stable now only works on Kubernetes clusters with load balancers.
• Use the fully qualified service name in the test jobs, so that hostname verification does not have to be skipped.

Part of #44

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• Integration tests passed (for non trivial changes)

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[labrenbe]

lgtm