Introduce Update Directory

[solracsf]

Allow to use a specific directory as updater working directory.

This is 100% backwards compatible as, if not set (the default), fallback to default datadirectory

• Param to be documented at config.sample.php at https://github.com/nextcloud/server
• Code to be also adjusted at https://github.com/nextcloud/server/blob/master/lib/private/Repair/MoveUpdaterStepFile.php#L46:

🆗 Done by nextcloud/server#33692

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This serves 2 purposes:

1. If datadirectory is set on a slow filesystems (like NFS) update can be very slow, or even time out. On our tests, gain is 500% less time on upgrade when moving away from NFS to Block, as working on very small files on network filesystems is a pain.
2. Allow sysadmin to specify where noisy I/O can be placed. If updatedirectory is placed on the same disk where application resides, which is not always the same disk as datadirectory, "Move new files in place" step is an atomic operation.