Watcher: Add API for time based throttling with expiry

[spinscale]

Describe the feature:

Currently watcher provides two mechanisms of throttling, see here. A watch or an action inside of a watch can be configured to be throttled for a certain amount of time, so that you do not send out emails every few minutes. Or a watch/action can be acknowledged via an API, so that it needs to become false first to ever fire again.

However, there is another use-case, where you would want to mix the two in order to silence a watch/action for a certain amount of time before the watch/action will be firing again.

A possible solution to this could be to extend the ACK Watch API to provide a body, that contains an expiry date for the watch or for certain actions.

Right you one can call

POST _xpack/watcher/watch/my_watch/_ack/action1,action2
POST _xpack/watcher/watch/my_watch/_ack

Adding a body like this

POST _xpack/watcher/watch/my_watch/_ack
{
  "expiry_date" : "2018-09-25T12:12:12Z"
}

POST _xpack/watcher/watch/my_watch/_ack
{
  "expiry_after" : "25m"
}

would add a time based throttling instead of an ack based one.

[elasticmachine]

Pinging @elastic/es-core-infra

[dakrone]

This has been open for quite a while, and hasn't had a lot of interest. For now I'm going to close this as something we aren't planning on implementing. We can re-open it later if needed.