Allow customization of the used ElasticsearchTransport.

[sothawo]

The Elasticsearch Java client library has the possibility to exchange its own implementation of the ElasticsearchTransport interface with a custom implementation provided by the user (implemented in elastic/elasticsearch-java#584).

Spring Data Elasticsearch should expose this possibility in its configuration class so that by default the implementation provided by Elasticsearch is used, but can be replaced by a user provided one.

#2604 addresses this topic on the lower level of the used http client. The point of this issue is add the possibility to replace the transport implementation, not to provide such implementations for different http client libraries.