CAMEL-8149: Better support for application-generated identifiers in bulk requests

https://issues.apache.org/jira/browse/CAMEL-8149

This design change allows upstream message processors to provide elasticsearch-java ActionRequest subclasses in the message body for more fine-grain control over the data that is sent to the es endpoint.
I'm wondering if it's better to do getBody() here with an instanceof instead. I'm concerned that with the type converters in place we'll try to coerce things when we shouldn't.