Support custom HCLOUD_ENDPOINT in cluster-autoscaler Hetzner manager

[kamushadenes]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds support to the HCLOUD_ENDPOINT environment variable, following what's used by other projects like https://github.com/hetznercloud/hcloud-cloud-controller-manager and https://github.com/hetznercloud/csi-driver.

Which issue(s) this PR fixes:

Fixes #7284

Special notes for your reviewer:

We have developed a soon-to-be-open-source proxy that forces specific labels in order to provide scoped API access. This support is required to make full use of it inside a Kubernetes cluster.

Does this PR introduce a user-facing change?

The HCLOUD_ENDPOINT environment variable is now supported to set a custom endpoint for Hetzner usage is cluster-autoscaler.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[linux-foundation-easycla]

• ✅login: kamushadenes / (143eea4)

The committers listed above are authorized under a signed CLA.

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[apricote]

Awesome! Very curious to see your proxy, feel free to submit it to our awesome-list once you are ready 🚀

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: apricote, kamushadenes

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Needs approval from an approver in each of these files:

• cluster-autoscaler/cloudprovider/hetzner/OWNERS [apricote]

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[kamushadenes]

Thanks for merging @apricote !

When can we likely expect a release containing this change?

[apricote]

This is currently in master, so will go out with Kubernetes 1.32.

I am not sure what the official policy is on backporting features to older release series.