AggregationStrategy - Allow to use a pojo with no Camel API dependencies

See also CAMEL-3671

When using the AggregationStrategy with the various EIPs, then its tied to the Camel API. We should allow people to use plain pojos, and have Camel call a method on the pojo. This would require a convention of the parameters of the method; so Camel knows what to pass as values.