Add a Validating Admission Webhook for the ScheduledComplianceScans CRDs

[AleksandarSavchev]

How to categorize this PR?

/area compliance
/kind enhancement

What this PR does / why we need it:
This PR introduces a new validating and mutating webhook for ScheduledComplianceScan resources to the diki-operator.

The mutating webhook does the following defaults for ScheduledComplianceScan resources:

• spec.schedule = "0 0 * * 0"
• spec.successfulScansHistoryLimit = 3
• spec.failedScansHistoryLimit = 1

The validating webhook does the following validation checks for ScheduledComplianceScan resources:

• checks for correct cron format in spec.schedule
• check history limits to not be negative integers
• denies updates to complianceScan template

Which issue(s) this PR fixes:
Fixes #38

Special notes for your reviewer:

Release note:

A webhook validating `ScheduledComplianceScan` resources has been added to the `diki-operator`.

A webhook mutating `ScheduledComplianceScan` resources has been added to the `diki-operator`.

[gardener-prow]

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