Add rank_feature, rank_features field datatypes and rank_feature query

Relates: #3836 

Provide support for 

- the new [`rank_feature`](https://www.elastic.co/guide/en/elasticsearch/reference/7.0/rank-feature.html), [`rank_features`](https://www.elastic.co/guide/en/elasticsearch/reference/7.0/rank-features.html) field datatypes
- [the new `rank_feature` query](https://www.elastic.co/guide/en/elasticsearch/reference/7.0/query-dsl-rank-feature-query.html)

