Add Rclone provider in backup destination creation workflow

[DavidePrincipi]

Implement new backup destination type "Rclone compatible provider" in the creation workflow, following the UI described in issue #7902:

UI/UX design

See https://www.figma.com/design/OsYqPXdkKJz0tBJRsiQgAf/NethServer-shared-file?node-id=4400-13382&t=tjGccaRHOV8MIySR-4

Change summary

• Remove "See details" modal.
• "Edit" is the default action for backup destinations.
• Encryption keys are no longer visible in the UI, and must be reused only through export/import cluster backup workflows.
• Password fields are not shown in Edit; empty means "Unchanged", consistent with SMTP routes in Mail module.
• Remove Azure provider (will be migratet to Rclone).
• Retain other providers, apply minor icon tweaks.
• For the new "Rclone configuration" destination:
  • Provide textarea for rclone config
  • Support .conf file upload
  • Add Path (optional) text input file
  • Allow destination name for UI
  • Advanced section for encryption key as existing types (hidden in Edit)
• In Edit mode: show form with empty textarea, without Advanced section

See also

• NS8 backup destination management #7902
• https://mattermost.nethesis.it/nethesis/pl/qwikfse81br39jtbkfjr4cdkeo