Schedule format with forward slash (/) not supported

Maybe I missed something, but it seems like I can't set a cron schedule that includes a forward slash like so:
```
*/10 * * * *
```
this should execute the cronjob at every 10th minute, so at :00, :10, :20, :30, :40 and :50.
I used the following command to update the crontask:
```
ee cron update 1 --schedule="*/10 * * * *"
```
and got the following error:
`Error: Schedule format should be same as Linux cron or schedule helper syntax(Check help for this)`

Incidentally, it is not possible to set multiple minutes either:
```
ee cron update 5 --schedule="0,10,20,30,40,50 * * * *"
```

The interesting part is when it comes to reading existing cronjobs. Running `ee cron list website.com` gives the following output:
```
+----+-------------+-------------------------------------+----------------+
| id | site_url | command | schedule |
+----+-------------+-------------------------------------+----------------+
| 5 | website.com | sh -c 'wp cron event run --due-now' | 0 5/10 * * * * |
+----+-------------+-------------------------------------+----------------+
```
The issue is that `0 5/10 * * * *` itself is not a properly formatted cron schedule expression, at least when validated with a website called [crontab.guru](https://crontab.guru/).
I suspect it should actually be `5/10 * * * *`, but then again this cannot be set.
So changing an existing cron schedule makes it impossible to go back…

Start by reproducing the issue with the `ee cron update` commands using `*/10 * * * *` and `0,10,20,30,40,50 * * * *`, then inspect the schedule handling behind `ee cron list`. Compare accepted input with standard five-field Linux cron syntax and the displayed `0 5/10 * * * *` value. Done means slash and comma schedules can be set and existing schedules round-trip in valid cron format.

Written by the indexing model from the issue text.