WIP: Enable configuration of HTTP client in ElasticSearch sink

[AndreLouisCaron]

It's quite common to deploy ElasticSearch behind some authenticating reverse proxy (e.g. with AWS ElasticSearch).

The original implementation directly instantiates org.apache.http.impl.client.DefaultHttpClient with no form of control over authentication.

This change enables configuring the HTTP client with any form of authentication.

NOTE: this is a draft change to get feedback. I will add proper tests once I get initial feedback about the design.