Support a custom transport client in camel-elasticsearch

The camel-elasticsearch client does not support a customizable TransportClient. It currently configures the client with hard-coded values when the endpoint is created.

This is problematic when fine-grain control over client creation is required. Also, this causes issues for the health of the camel build, as the current unit tests use a local client as a result which will spin up a full elasticsearch server during the build instead of relying on a mock object for the tests.