added possibility to retrieve hcloud cluster config from file

[karsten42]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Picking up from this issue it allows to set the the Hetzer Cloud Cluster Config via a file instead of an environment variable.
As mentioned in the issue the cluster config can become too long for an env var (happened to me today).

Which issue(s) this PR fixes:

Fixes #7652

Does this PR introduce a user-facing change?

- allow setting Hetzner Cloud Cluster Config by providing a json config file, set via `HCLOUD_CLUSTER_CONFIG_FILE`

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: karsten42
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[k8s-ci-robot]

Hi @karsten42. Thanks for your PR.

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