feat(ske): add ephemeral ske kubeconfig

[h3adex]

Description

This PR adds the stackit_ske_kubeconfig ephemeral resource. The default expiration has been reduced to 30 minutes. Since these credentials only need to persist for the duration of a Terraform run, where even slow Helm deployments rarely exceed a 10-15 minute window, a 60-minute expiration is excessive.

Tested example code:
https://professional-service.git.onstackit.cloud/professional-service-best-practices/professional-service/pulls/18

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[h3adex]

don't review yet. E2E tests still open

[h3adex]

E2E Tests

[github-actions]

This PR was marked as stale after 7 days of inactivity and will be closed after another 7 days of further inactivity. If this PR should be kept open, just add a comment, remove the stale label or push new commits to it.

[github-actions]

This PR was marked as stale after 7 days of inactivity and will be closed after another 7 days of further inactivity. If this PR should be kept open, just add a comment, remove the stale label or push new commits to it.

[github-actions]

This PR was marked as stale after 7 days of inactivity and will be closed after another 7 days of further inactivity. If this PR should be kept open, just add a comment, remove the stale label or push new commits to it.

[rubenhoenle]

@h3adex Please resolve the comments you've already adressed 😅

[h3adex]

@rubenhoenle done

[rubenhoenle]

There are conflicts @h3adex

[h3adex]

@rubenhoenle resolved conflicts

[h3adex]

Resolved last comment and squashed commits