Create Side input load test for Java SDK

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

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

Imported from Jira [BEAM-5982](https://issues.apache.org/jira/browse/BEAM-5982). Original Jira may contain additional context.
Reported by: ŁukaszG.
This issue has child subcomponents which were not migrated over. See the original Jira for more information.
Subtask of issue #19230

Read the linked proposal and parent issue #19230 first, since the issue says the child subcomponents and additional context were not migrated. Identify the Java SDK load-test entry points involving SyntheticStep and Synthetic source; done means implementing the Side Input load test that creates pipeline load as described there.

Written by the indexing model from the issue text.