Implement `--mark-skipped` on the backfill command

Seems loosely related to a few tickets, like #262 which was closed without explanation. From what I am gathering, following the different links across tickets, everything to do it might have been implemented. Everything but the UI, making it available to end users. **Use case / motivation** There exists a `--mark-success` option for the `dags backfill` command but no such `--mark-skipped` option. Marking a task as success is not equivalent to marking it has skipped, when it comes to later on knowing what happened, e.g. why some data might not have been computed and be missing. <!-- What do you want to happen? I would expect the behavior to be the same as marking the tasks has success, only the state would be different, tasks would be marked as skipped. --> **Are you willing to submit a PR?** I can try. Maybe some pointers could help. **Related Issues** #262 (closed without being implement, applied to the Web UI, not the CLI) #1155 the ticket suggesting this is possible.