[feat] Clone mailbox feature

Considering sometimes passw rekey fails (and a 'revert' feature would be appreciated) and that i prepare mailboxes for my users with sieve rules and special settings and folders:

>Would it be possible to completely CLONE the mailbox and all settings with another new alias name?

It may also serve like a kind of temporary mailbox backup before doing heavy changes/migrations.

The issue names no files, tests, or entry points. Start by locating the existing mailbox creation, alias, settings, Sieve-rule, folder, and rekey flows, then clarify the scope and define how a completed clone and any backup or revert behavior should be verified.

Written by the indexing model from the issue text.