Enhancement: Add flag to source the backup from a program's standard output

The `backup` command now supports sourcing the backup content from the standard
output of an arbitrary command, ensuring that the exit code is zero for a
successful backup.

Example: `restic backup --stdin-from-command mysqldump [...]`

https://github.com/restic/restic/issues/4251
https://github.com/restic/restic/pull/4410