[FEATURE][UI] Fill in the secret and secret namespace when restoring the BackupBackingImage

[ChanYiLin]

Is your feature request related to a problem? Please describe (👍 if you like this request)

After this feature is closed: #8884
Longhorn stores the encryption secret and secret namespace information in the BackupBackingImage now, so we can help users to fill in the information when they restore the BackupBackingImage.

Describe the solution you'd like

Previously, we let users to fill in the secret and secret namespace when they want to restore the BackupBackingImage.
But now we should help users to fill in the secret and secret namespace to improve the user experience.

Describe alternatives you've considered

.

Additional context

cc @a110605

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at:

• Create encrypted and non-encrypted backing images
  https://longhorn.io/docs/1.7.2/advanced-resources/backing-image/backing-image-encryption/
• Backup backing images
• Restore encrypted backing images: auto-fill secret and namespace in the pop-up
  
• Restore non-encrypted backing images: hide secret and namespace inputs in the pop-up
  

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at [IMPROVEMENT] backup backing image should store the secret and secret namespace if it is encrypted #8884

• If labeled: area/ui Has the UI issue filed or ready to be merged (including backport-needed/*)?
  The UI issue/PR is at feat: fill in the secret and secret namespace when restoring the BackupBackingImage longhorn-ui#822

• If labeled: require/doc Has the necessary document PR submitted or merged (including backport-needed/*)?
  The documentation issue/PR is at doc(1.8.0): update backing image restore UI website#1020

[chriscchien]

Verified pass on longhorn master(longhorn/longhorn-ui@944339c) with test steps