Allow secret to be referenced

[cbarbian-sap]

Currently, the replicated secret content is defined through spec.template. It would be useful to have a second way through a reference to an existing regular secret that would be replicated, such as spec.secretRef. Such that exactly one of spec.template and spec.secretRef must be provided. A little extra logic might be required in the case that one of the target secrets equals the referenced source secret; in that case, probably, the replication simply should be skipped.