Add NuxeoJavaClient/x.y.z in User-Agent header

Currently the client sends header User-Agent: okttp3/3.9.1, which is the default header sent by the okhttp library we use.

It would be useful, for diagnostic/monitoring/debugging purposes to add an indication that the connection comes from the Nuxeo Java Client, like for instance: User-Agent: okttp3/3.9.1 NuxeoJavaClient/3.0.2