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 (y), I can start working on it.

Imported from Jira [BEAM-10838](https://issues.apache.org/jira/browse/BEAM-10838). Original Jira may contain additional context.
Reported by: clandry94.