Data stream settings api experiment

[masseyke]

NOTE: This is just a draft.
This is a prototype of an API that updates the settings on a data stream, and looks like:

PUT /_data_stream/my-data-stream*/_settings
{
  "index": {
    "number_of_shards": 3,
    "lifecycle.name": "new_lifecycle"
  }
}

[elasticsearchmachine]

Hi @masseyke, I've created a changelog YAML for you.