Build a cross-platform JAR from the Java bindings

[BudgieInWA]

The Java bindings implemented in the osm2streets-java crate are functioning (see #68) and being used in the dev build of 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.

[dabreegster]

Haven't looked into it, but possibly useful new crate, and some of the comments are talking about how to do related things (like load the JNI lib at runtime): https://old.reddit.com/r/rust/comments/11no9eg/announcing_rustmavenplugin_compile_and_bundle/