[FEATURE] Allow CRDs to target an `OpenSearchCluster` in another namespace

[tmsdce]

Is your feature request related to a problem?

I'm just starting off with the operator so maybe I'm wrong but it seems that all CRs can only target an opensearch cluster in the same namespace in which they are declared. It does not seem possible to declare say an OpensearchUser in namespace A targeting an OpenSearchCluster deployed in namespace B. The CRDs spec seems to confirm this as there is no namespace field available for targeting a cluster :

# kubectl explain opensearchusers.spec.opensearchCluster

GROUP:      opensearch.opster.io
KIND:       OpensearchUser
VERSION:    v1

FIELD: opensearchCluster <Object>

DESCRIPTION:
    LocalObjectReference contains enough information to let you locate the
    referenced object inside the same namespace.
    
FIELDS:
  name	<string>
    Name of the referent.
    More info:
    https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#names
    TODO: Add other useful fields. apiVersion, kind, uid?

What solution would you like?

Being able to target an OpenSearchCluster regardless of the namespace it is deployed to.

• This is particularly useful when you want a single Opensearch cluster managing multiple tenants instead of having to deploy a complete opensearch cluster per tenant which can be expensive resource-wise even for small clusters and adds maintenance burden
• This would also allow application owners to handle their own configuration (users, index templates...) while letting "cluster administrators" handle the lifecycle of the Opensearch cluster itself

What alternatives have you considered?

Deploying an opensearch cluster per tenant and/or application instance which seems overkill in some cases

Do you have any additional context?

For reference, the RabbitMQ operator does this. For example, the Vhosts CRD gives the ability to specify the target namespace of the RabbitmqCluster (cf. https://github.com/bitnami/charts/blob/main/bitnami/rabbitmq-cluster-operator/crds/messaging-topology-operator/rabbitmq.com_vhosts.yaml#L59)

[swoehrl-mw]

Hi @tmsdce. There was some discussion about this topic in #596.
Bottom line: Cross-namespace references will likely not be supported as the namespaces are a security boundary and opening that up either breaks security or requires us to implement extra logic and configuation to properly manage it.

[tmsdce]

Hi @swoehrl-mw
Thanks for your prompt reply. Indeed, I didn't catch #596, sorry for the noise. We can consider the present issue as a duplicate so I'm closing it