SecretSyncController warmup for cluster-scope

[burmanm]

What is missing?

Follow up to this issue:

#1399

This functionality is used in the warmup phase of the SecretController, in initializeCache(). This works now for single-namespace as well as multi-namespace, but not for cluster-scoped installation requiring all namespaces as sync source.

That requires a bit more work and could be done by first fetching K8ssandraCluster objects (cluster-wide) and then listing their namespaces and after that fetching the required secrets. Fetching all the secrets and then parsing them could return huge amount of resources, so that would reduce the burden to the Kubernetes cluster.

Why do we need it?

Environment

• K8ssandra Operator version:

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  **Anything else we need to know?**:

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: K8OP-250