clean up stackit client

[aniruddha2000]

How to categorize this PR?

/kind cleanup

What this PR does / why we need it:

This PR introduces clean up for the stackit client and the mocks.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Breaking changes:

[ske-prow]

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[ske-prow]

PR needs rebase.

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[ske-prow]

@aniruddha2000: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cloud-provider-stackit-verify	fd6f2fb	link	true	/test pull-cloud-provider-stackit-verify

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see the gardener testing guideline for how to avoid and hunt flakes.

Details

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