Improve UI UX around synchronised secrets

[richard-cox]

• 2.10.0, 2.9.3 and 2.8.9 introduced a rather nice way to sync a single upstream secret to multiple downstream clusters
  • [Backport v2.8] Enhance the secret-synchronization controller from Rancher's management cluster -> downstream clusters  rancher#47470
• There is however some UX issues around this that need to be addressed
• Some issues from top of head
  • user cannot easily use the feature
    • currently all via four special annotations
  • it's not clear to the user
    • if they delete a secret upstream that it could break downstream clusters
    • a deleted secret downstream would just reappear etc
  • Unclear how secrets synced to downstream clusters are cleaned up when the dependent clusters are deleted
• Some requirements from top of head
  • user should be able to easily configured a secret to sync with specific downstream clusters
  • it should be clear to a user that a secret is synced with specific downstream clusters
  • it should be clear to a user that a secret is being synced from the upstream cluster
  • it should be easy to maintain an upstream secret that is synced to downstream clusters
    • related to validity of downstream clusters to sync to

For more context see the Rancher v2.8.9 release note PR