[FEATURE] Shrink API observable helper to track progress

[Mpdreamz]

We already support the Shrink API through ShrinkIndex()

https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-shrink-index.html

However tracking its progress involves calling monitoring API's until it has no relocating shards anymore.

It be nice if we expose this through an Observable like we do with other coordinated requests.