Ability to cancel job from the queue?

Hi @khmarbaise ,

I've looked through the code and don't see any way of canceling an item while it's still in the queue. Is this something I can look to add?

No files, tests, or entry points are named. Start by locating queue-related operations in the Java client and checking how Jenkins exposes cancellation for queued items. Define the supported cancellation behavior and add coverage; the issue is complete when a queued job can be canceled through the client.

索引モデルが issue の本文から書いたものです。