Add addmission webhook for shoot object

[dergeberl]

How to categorize this PR?

/kind enhancement
/cc @stackitcloud/ske-infrastructure

What this PR does / why we need it:

We already have validation functions for the ControlPlaneConfig and InfrastructureConfig but we had no validation webhook for the shoot object. This PR adds the webhook.

It also fixes the InfrastructureConfig as this was not working, as it is possible to habe no InfrastructureConfig.Networks.Workers if InfrastructureConfig.Networks.ID is set.

I created a cluster with and without SNA via the ske-api.

We want to use this for the ALB integration #54

Special notes for your reviewer:

Please also check the validation itself as this was not used before and needed already adoption in the InfrastructureConfig part.

[ske-prow]

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[ske-prow]

@dergeberl: 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-gardener-extension-provider-stackit-integration-tests	9b8a7b5	link	true	/test pull-gardener-extension-provider-stackit-integration-tests

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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