Add support for separate Kubernetes cluster SecretStore

[mcavoyk]

Describe the solution you'd like
Add SecretStore for secrets store in another cluster (namespace or cluster-wide).

Describe alternatives you've considered
Moving secrets to cloud provider secret and each each cluster having a SecretStore which has authentication to the cloud provider store.

Additional context
Our use-case is that due to network partition, one cluster has access to a secret backend, but other clusters do not. The cluster with network access to the secret backend is accessible to other clusters, so this could provide a link if needed to secrets from the other cluster.

May need more thought, some ideas around only accessing explicit Secrets which already exist, or whether relaying ExternalSecrets is allowed (e.g. ExternalSecret in Cluster B creates new ExternalSecret in cluster A which which has access, secret is propogated to cluster B to be used by workloads).

/kind feature