Add validating webhook implementation for KafkaUser resource

[panyuenlau]

Problem Statement

Currently there is no validations against KafkaUser resources Create/Update/Delete operations.

Proposed Solution

Add a webhook implementation to validate resource creation and update for the KafkaUser resource. Some of the scenarios that we should reject client requests:

• Referenced KafkaCluster doesn’t exist in the K8s cluster

• Referenced KafkaCluster is being deleted

• When spec.TopicGrants is set:

  • But the referenced KafkaCluster doesn’t have ACL enabled
  • The referenced KafkaCluster has ACL enabled, but the TopicGrants[x].topicName refers to a non-existing topic in the Kafka cluster

Additional Context

WIP PR: #968, will take some time to finish up the PR