Access indexing service using a proper web client

`org.fao.geonet.searching.controller.EsHttpProxy` uses manual http connection handling to query the indexing service at the URL defined by `${gn.index.url}`.

In order for the app to be "cloud native", it must do so in a way that supports automatic client-side load balancing, so that the value of `${gn.index.url}` can be the name of the indexing service, and the load-balanced http client can discern which service instance to hit (usually it has a cache of service instances retrieved from discovery-service).

The easiest way of achieving so is using spring's `WebClient` (see https://spring.io/guides/gs/spring-cloud-loadbalancer/)
The recommended way is to create an [OpenFeign](https://docs.spring.io/spring-cloud-openfeign/docs/2.2.5.RELEASE/reference/html/) client for the target service, which under the hood will use `WebClient`