Create Combine load test for Java SDK

This is more thoroughly described in this proposal: https://docs.google.com/document/d/1PuIQv4v06eosKKwT76u7S6IP88AnXhTf870Rcj1AHt4/edit?usp=sharing 

In short: this ticket is about implementing the Combine load test that uses SyntheticStep and Synthetic source to create load on the pipeline.