Add support to "Tasks", eventually

[2785]

https://docs.gitlab.com/ee/user/tasks.html <- tasks described here.

This thing is currently available via gitlab UI & it fires off webhooks with object kind work_item.

No available API on top of this yet, not sure how stable the entity schema is, but we could probably support that when there's officially an API over it.

What's mildly annoying here with gitlab is that - these work_item webhooks come with a X-Gitlab-Event header that says it's an Issue Hook - and go-gitlab's recommended method of parsing webhook kinda breaks down on this in attempting to parse it as an issue event. 🤷

All in all, more of a "hey gitlab did this fancy new half as**d thing that breaks some very specific existing stuff". I'm down to write some code when gitlab publishes the API to add support to the library.

[svanharmelen]

Let's see how the API turns out when it's made public 😏

[svanharmelen]

Is this still an open issue, or should we close this one for now?