feat: support user migration

[ChristophWurst]

MVP implementation of #12001

To do

• Migrate basic info (email, name)
• Migrate server settings (imap, smtp, auth)
• Migrate aliases
• Unit tests
• Integration tests

How to test

1. Install and enable https://github.com/nextcloud/user_migration
2. Either use the personal settings or occ to export/import

It will look something like this:

hint: run cron. it's a background job that won't make progress otherwise.

or via the CLI:

$ occ user:export -t mail_account user1
Exporting user information in user.json…
Exporting settings in settings.json…
Exporting versions in versions.json…
Export saved in /projects/nextcloud/admin_2025-12-12_14-24-31.zip
$ occ user:import --user user2 /projects/nextcloud/admin_2025-12-12_14-24-31.zip
Importing from /projects/nextcloud/admin_2025-12-12_14-24-31.zip…
Importing user information from user.json…
Importing settings from settings.json…
No version for OCA\DAV\UserMigration\CalendarMigrator, skipping import…
No version for OCA\DAV\UserMigration\ContactsMigrator, skipping import…
No version for OCA\Files_Trashbin\UserMigration\TrashbinMigrator, skipping import…
No version for OCA\Settings\UserMigration\AccountMigrator, skipping import…
No version for migrator files (OCA\UserMigration\Migrator\FilesMigrator), skipping import…
Successfully imported admin
Successfully imported from /projects/nextcloud/admin_2025-12-12_14-24-31.zip

[ChristophWurst]

@kesselb @SebastianKrupinski please review so I can finish the task this week

[SebastianKrupinski]

Tested. Export and Import. Working

The only change I would make if to but in some export and import messages for console use.

At the moment there is no indication that the mail app settings where processed