M1: key management, backup locations and occ commands

- migration: ngb_targets, ngb_runs, ngb_snapshots
- KeyService: master key stored encrypted with the instance secret
  (unattended backups), passphrase-wrapped copy in every repository and
  in the recovery kit, recorded admin confirmation per key fingerprint
- TargetService: locations built from files_external backends without a
  mount; files_external is loaded through IAppManager::loadApp() and its
  enabled state is left alone; credentials encrypted with ICrypto;
  existing repositories are adopted only if made with this key, and a
  repository cannot be registered twice
- Repository: initWithKey/openWithKey with an encrypted key check
- occ: backup:key:init|kit|confirm, backup:target:backends|add|list|remove

appinfo/info.xml

@@ -16,4 +16,13 @@
         <lib>sodium</lib>
         <nextcloud min-version="32" max-version="35"/>
     </dependencies>
+    <commands>
+        <command>OCA\NgBackup\Command\KeyInit</command>
+        <command>OCA\NgBackup\Command\KeyKit</command>
+        <command>OCA\NgBackup\Command\KeyConfirm</command>
+        <command>OCA\NgBackup\Command\TargetBackends</command>
+        <command>OCA\NgBackup\Command\TargetAdd</command>
+        <command>OCA\NgBackup\Command\TargetList</command>
+        <command>OCA\NgBackup\Command\TargetRemove</command>
+    </commands>
 </info>