feat: add validating admission webhooks for Ingress and IngressClass annotations

[kamilprzybyl]

This PR introduces two separate Validating Admission Webhooks for the STACKIT ALB controller to enforce constraints, formatting, and allowed values on ALB annotations.

By failing fast at the Kubernetes API level, this prevents invalid configurations from being created in the cluster if they do not meet the ALB controller's requirements.

[ske-prow]

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[kamilprzybyl]

.

[kamilprzybyl]

When the webhook pod runs with hostNetwork: false, the API server times out trying to reach it (context deadline exceeded). It works fine when hostNetwork is set to true.

Should we stick with hostNetwork: true as a workaround, or is there a preferred way to enable API-server-to-Pod routing for webhooks in SKE?

[ske-prow]

@kamilprzybyl: 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	51071c5	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.

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[dergeberl]

/hold
We decided to merge #879 first and rebase this PR to main.