Integrate OAuth Authorization into Strimzi

[ppatierno]

This issue is about integrating into Strimzi the OAuth Authorization provided by Keycloak.
It's related to this strimzi/strimzi-kafka-oauth#24 and providing the right CRD additions into the Kafka resource for enabling this type of authorization (other than the simple one already in place).
Authorization should have a new keycloak type and the related configuration parameters in the CRD.
Following a proposal for that.

# ...
authorization:
  type: keycloak
  tokenEndpointUri: <URL of the Keycloak's token endpoint>
  clientId: <A client id of the OAuth client definition in Keycloak>
  delegateToKafkaAcls: false #if fallback to Simple authz if no access provided by Keycloak
  tlsTrustedCertificates:
     - secretName: oauth-server-cert
       certificate: ca.crt
  disableTlsHostnameVerification: false
  superUsers:
    - Alice
    - CN=Bob

[ppatierno]

@mstruk @scholzj any thoughts?

[mstruk]

Yes, looks good.
Maybe we should also support

 clientSecret:
   secretName: my-cluster-oauth
   key: clientSecret

But currently KeycloakRBACAuthorizer doesn't support it, so maybe we add it later.

[ppatierno]

I forgot superUsers field, just added.