normalized SSL config not applied when LS core uses private-API build_client directly

[yaauie]

Logstash core uses this plugin build_client to build an Elasticsearch client that it can use for communications with Elasticsearch in core features like Monitoring and Central Management.

Unfortunately, the SSL standardization work that was completed for 11.14.0 of this plugin in #1118 was not aware of Logstash-core's use of this private API, which bypasses the plugin's normal #register method. Because #register is not sent by Logstash core, the ls-core-provided path-to-trust directives like cacert and truststore are not available to the client that it builds, leading to obtuse PKIX-path building failures.