WIP: Support for explicitly configuring Image for Hetzner Machines

[ahmedwaleedmalik]

What this PR does / why we need it:
This PR adds an option on the dashboard to configure the image for Hetzner machines.

Which issue(s) this PR fixes:

Fixes #6926

What type of PR is this?

/kind feature

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

Support for explicitly configuring Image for Hetzner Machines

Documentation:

NONE

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kubermatic-bot]

@ahmedwaleedmalik: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pre-dashboard-api-unit	3de71ed	link	true	/test pre-dashboard-api-unit
pre-dashboard-api-e2e	3de71ed	link	true	/test pre-dashboard-api-e2e

Full PR test history

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