Add support for Maven buildsystem.

[sebastic]

As reported on the Debian GIS mailinglist, the switch to gradle 8.x broke the osmosis Debian package build because gradle is stuck at 4.4. It's nearly impossible to package recent gradle releases due to its use of kotlin which in turn depends on ancient JREs. Fedora has removed their gradle package entirely because it's unmaintainable for example.

This PR adds support for Maven to build the JARs. This helps packagers keep osmosis in their distribution.

I haven't found a good replacement for the gradle application plugin to create the launcher, so I've reverted back to plexus in the Debian package. We should likely move those changes back into the osmosis repo as well.