Build a cross-platform JAR from the Java bindings

The Java bindings implemented in the `osm2streets-java` crate are functioning (see #68) and being used in the dev build of [JOSM2Streets](https://github.com/BudgieInWA/JOSM2Streets/).

In order to distribute the JAR though, builds targetted for all the platforms we want to support need to be bundled, and the appropriate one needs to be chosen for extraction.

The Java class should probably try to load the library from the system paths first, so that individual users can select their own version of the library by placing it on the system path. This will be useful for faster iterating while developing osm2steets too.