Provide a configuration property for Elasticsearch path prefix

[TarekSaid]

As of Spring Boot 2.4.2, with spring-data if the ES server is behind a reverse proxy with a prefix, there's no way to use Spring Boot's autoconfiguration for spring.elasticsearch.rest or spring.data.elasticsearch.client.reactive properties. One has to use the AbstractElasticsearchConfiguration in order to use the ClientConfiguration.builder().withPathPrefix(prefix). The horror.

Ideally, we could:

• directly specify the path-prefix in the spring.elasticsearch.rest.uris property (useful in case there are multiple urls, each with its own path prefix). I don't know how that would work with spring.data.elasticsearch.client.reactive.endpoints, as it only uses a host:port format.
• specify the path-prefix in a spring.elasticsearch.rest.path-prefix or spring.data.elasticsearch.client.reactive.path-prefix property. In that case, it would behave like the withPathPrefix(String) method, applying the prefix to all the uris/endpoints in the list

[wilkinsona]

If we need a new property, we need to consider #23106.

[philwebb]

We'll either add a property when we look at #23106 or we might consider adding a Customizer callback interface.

[wilkinsona]

On the imperative side, the path prefix can be configured using a RestClientBuilderCustomizer bean. On the reactive side there's no equivalent customizer. For symmetry, it feels to me like we should add one regardless of whether or not we add a new path-prefix property.

[wilkinsona]

directly specify the path-prefix in the spring.elasticsearch.rest.uris property (useful in case there are multiple urls, each with its own path prefix)

Elasticsearch's REST client does not appear to support this. The URIs are used to create Node or HttpHost instances, neither of which can convey a path prefix. In other words, the path prefix has to be the same across every node with which the client will communicate. Given this, I think that the only configuration property-based option that makes sense is to provide a new path-prefix property. This will work for both clients.