[control-plane]: Support auto generation of KafkaSource identity service account and expose in AuthStatus

[creydr]

Problem

As the Eventing OIDC feature track describes, the AuthStatus is meant to provide the generated service account name in the resource status.

When knative/eventing#7173 is done, we should:

• when the feature flag (see Add authentication.oidc feature flag knative/eventing#7174) is disabled:
  • no change in behavior
• when the feature flag (see Add authentication.oidc feature flag knative/eventing#7174) is enabled:
  • create the service account for the KafkaSources identity
  • expose the service accounts name in .status.auth.serviceAccountName

Additional Information:

• See implementation for trigger in eventing-core: Support auto generation of Triggers identity service account and expose in AuthStatus knative/eventing#7222