Enhancement: Support reading backup from a program's standard output

When reading data from stdin, the `backup` command could not verify whether the
corresponding command completed successfully.

The `backup` command now supports starting an arbitrary command and sourcing
the backup content from its standard output. This enables restic to verify that
the command completes with exit code zero. A non-zero exit code causes the
backup to fail.

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

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