Add an API for adding and removing indices from a data stream

[dakrone]

Similar to the Aliases API, it would be useful to have an expert-level API for adding and removing indices from a data stream. So, copying from the aliases API, something similar to:

POST /_data_stream
{
  "actions": [
    {
      "add": {
        "index": "my-index",
        "data_stream": "my-data-stream"
      },
      "remove": {
        "index": "my-other-index",
        "data_stream": "my-data-stream"
      }
    }
  ]
}

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[dakrone]

@danhermann I think this can be closed since your PR was merged right?

[danhermann]

The REST endpoint is still outstanding.