Make expansion service Java launcher configurable in pipeline options.

We need Java to run expansion services in subprocesses [1]. Currently this code always uses the "java" executable on the $PATH. But changing $PATH or adding an alias is a global(ish) change, and it's cumbersome, or even impossible in some environments (Forge). Having a pipeline option for it is easier and better isolated.

[1] https://github.com/apache/beam/blob/master/sdks/python/apache_beam/utils/subprocess_server.py

Imported from Jira [BEAM-11414](https://issues.apache.org/jira/browse/BEAM-11414). Original Jira may contain additional context.
Reported by: ibzib.