Add a withSSLContext builder method to ElasticsearchIO

Currently, the ElasticsearchIO transforms have only one way to securely read/write Elasticsearch by using the withKeystorePath builder method and providing the location of a keystore containing a client key in jks format.

This is a bit limiting, especially for Elasticsearch users not depending on shield to secure their clusters. I'd like to propose the addition of the builder method withSSLContext(SSLContext sslContext, which delegates to `httpClientBuilder.setSSLContext`.

If this is , I can start working on it.