Expose global retention settings via data stream lifecycle API

[gmarouli]

In this PR we expose the global retention via the GET _data_stream/{target}/_lifecycle API.

Since the global retention is a main feature of the data stream lifecycle we chose to expose it by default.

GET /_data_stream/my-data-stream/_lifecycle
{
 "global_retention": {
      "default_retention": "7d",
      "max_retention": "365d"
  }, 
  "data_streams": [...]
}