Allow multi namespace cache scoping

[ActuallyTrent]

Note to reviewers: remember to look at the commits in this PR and consider if they can be squashed

Summary Of Changes

Allows scoping the cache namespace to more than 1 namespace if OPERATOR_SCOPE_NAMESPACE is a comma delimited list.

Additional Context

If someone needed to configure their RBAC with non-cluster roles and rolebindings they would be unable to run the operator. This project requires access to a lot of resources and not everyone's security stance allows for using clusterroles.

Without providing OPERATOR_SCOPE_NAMESPACE and using role instead of clusterrole you get errors such as

failed to list *v1.ConfigMap: configmaps is forbidden: User "system:serviceaccount:rabbit:rabbit-cluster-operator" cannot list resource "configmaps" in API group "" at the cluster scope

and a similar error if a resource is added in a namespace not covered by OPERATOR_SCOPE_NAMESPACE.

Without this change, it's only possible to scope the cache to a single namespace, and this allows for scoping to a list of namespaces.

Local Testing

Please ensure you run the unit, integration and system tests before approving the PR.

To run the unit and integration tests:

$ make unit-tests integration-tests

You will need to target a k8s cluster and have the operator deployed for running the system tests.

For example, for a Kubernetes context named dev-bunny:

$ kubectx dev-bunny
$ make destroy deploy-dev
# wait for operator to be deployed
$ make system-tests

[Zerpet]

Verified the new behaviour, working as expected. I did the following:

1. Create NS rabbit-1 and rabbit-2
2. Modify RBAC to use Role and RoleBinding, instead of is Cluster* variants
3. Deployed RBACs to NS rabbit-1 and rabbit-2
4. Started Operator using:

   make just-run OPERATOR_SCOPE_NAMESPACE="rabbit-1,rabbit-2"

5. Created a RabbitmqCluster object in NS rabbit-1
6. Observe the Operator reconcile with no errors
7. Created a RabbitmqCluster in default NS
8. Observe the Operator does not reconcile the object
9. Created a RabbitmqCluster in NS rabbit-2
10. Observe the Operator reconcile the object

[Zerpet]

The CI failure seems unrelated to this PR. I suspect it is failing because the PR is coming from a fork, and GitHub Secrets are not injected in forks, based on this message:

By default, secrets are not passed to workflows triggered from forks [...]