Add occ command to manually decrypt files

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For backup recovery scenarios of individual files it might be useful to have a simple command that allows to decrypt a file that was encrypted through SSE by providing the file + key files + instance secret. While there is https://github.com/syseleven/nextcloud-tools it is not really suitable for such scenarios, especially with object storage as it relies on the files still being present in the data directory and is using a filecache dump.

Roeland had the idea to implement such a command a while ago that could just be used by getting all files required to some temporary location and then running such a command.

The issue names no files, tests, or command entry point. Start by tracing Nextcloud's SSE encryption handling and existing CLI commands, then define the required file, key files, and instance secret inputs. Done means a documented command can decrypt an individual backed-up file, including files stored through object storage.