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

The `backup` command now supports the `--stdin-from-command` option. When using
this option, the arguments to `backup` are interpreted as a command instead of
paths to back up. `backup` then executes the given command and stores the
standard output from it in the backup, similar to the what the `--stdin` option
does. This also enables restic to verify that the command completes with exit
code zero. A non-zero exit code causes the backup to fail.

Note that the `--stdin` option does not have to be specified at the same time,
and that the `--stdin-filename` option also applies to `--stdin-from-command`.

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

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