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 the `--stdin-from-command` option. When using
this option, the arguments to `backup` are interpreted as a command. `backup`
then executes the command and stores its standard output in the backup. 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