Add `features` parameter to Get Index API

[yuliacech]

🐛 Wrong type

The Get Index API has been updated to expose the parameter features to avoid computing some of the index properties when requesting the API. It would be great to add this option to the IndicesGetRequest spec. The value can be one of aliases, mappings or settings or a combination of those. The default is aliases,mappings,settings.

[philkra]

@yuliacech thank you very much for reporting the missing property, I opened the PR #1562 to address this change. The release of the client will contain the new property.