Support unsealing into multiple namespaces

**Which component**:
Controller

**Is your feature request related to a problem? Please describe.**
We have several containers that needs fetching from a password protected registry - and currently we have to have the SAME secret defined for each namespace we need them in.

We also have other secrets, which we have to duplicate in multiple namespaces

**Describe the solution you'd like**
If we could put multiple namespace targets on 1 secret - that would make life so much easier :)

The issue identifies the Controller as the component to investigate; start by reading how it handles a sealed secret and its namespace target. Trace the current single-namespace flow and existing controller tests, then define completion as supporting multiple namespace targets from one secret without requiring duplicated secrets.

Written by the indexing model from the issue text.