Add support for Adiantum encryption (for devices lacking AES-NI acceleration)

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This would greatly improve encryption/packing speeds on devices without onboard AES acceleration, such as the Raspberry Pi. I find my Pi 4 performs extraordinarily well in all regards within Nextcloud except for the Backup app as it falls back to aes-256-cbc. The performance of Adiantum (especially ChaCha12) on the Pi 4 is more than enough to keep up with USB 3.0 drive, where I have Raspberry Pi OS installed, meaning encryption speeds would no longer be a bottleneck.

Edit: Adiantum has been a part of the kernel since 5.0, and 5.10 on the Raspberry Pi. Most modern devices likely have this cipher as an option.