[Feature] Introduce validating webhook

[timuthy]

Feature (What you would like to be added):
Like other components in the gardener org, Etcd-Druid should offer a validating webhook for the v1aplha1.druid.gardener.cloud API.

• Add boilerplate for validating webhook (code + Validatingwebhookconfiguration)
• Execute validation code in api/validation
• Extend validation code for etcd resource:
  • Validate value input, i.e. forbid invalid values
  • Validate immutable fields and forbid updates, esp. the ones that are also immutable in statefulsets (see [BUG] Statefulset updation fails as druid attempts to update forbidden fields #404)
  • Add WaitForFirstConsumer validation as highlighted in Zone pinning not working for volumes in Immediate binding mode gardener#8563

Motivation (Why is this needed?):
Primarily, a proper input validation prevents unexpected runtime failures.

[shreyas-s-rao]

/assign

[shreyas-s-rao]

/unassign

[unmarshall]

Add WaitForFirstConsumer validation as highlighted in g/g#8663

[shreyas-s-rao]

/assign @renormalize